Implement handling of WaterML 2.0 no data values

[EHJ-52n]

Implement handling of missing data in MeasurementTVPs:

<wml2:point>
<wml2:MeasurementTVP>
  <wml2:time>2019-07-08T14:55:00.000+02:00</wml2:time>
  <wml2:value xsi:nil="true"/>
  <wml2:metadata>
    <wml2:TVPMeasurementMetadata>
      <wml2:nilReason xlink:href="missing"/>
    </wml2:TVPMeasurementMetadata>
  </wml2:metadata>
</wml2:MeasurementTVP>
</wml2:point>

Complete GetObservationResponse

<?xml version="1.0" encoding="UTF-8"?>
<sos:GetObservationResponse xmlns:sos="http://www.opengis.net/sos/2.0" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.opengis.net/waterml/2.0 http://schemas.opengis.net/waterml/2.0/waterml2.xsd http://www.opengis.net/waterml/2.0 http://schemas.opengis.net/waterml/2.0/timeseries.xsd http://www.opengis.net/sos/2.0 http://schemas.opengis.net/sos/2.0/sos.xsd">
  <sos:observationData>
    <om:OM_Observation xmlns:om="http://www.opengis.net/om/2.0" xmlns:gml="http://www.opengis.net/gml/3.2" xmlns:wml2="http://www.opengis.net/waterml/2.0" gml:id="o_1">
      <om:type xlink:href="http://www.opengis.net/def/observationType/OGC-OM/2.0/OM_Measurement"/>
      <om:phenomenonTime>
        <gml:TimeInstant xmlns:gml="http://www.opengis.net/gml/3.2" gml:id="phenomenonTime_1">
          <gml:timePosition>2019-07-08T14:55:00.000+02:00</gml:timePosition>
        </gml:TimeInstant>
      </om:phenomenonTime>
      <om:resultTime xlink:href="#phenomenonTime_1"/>
      <om:procedure xlink:href="Hydrometric_Station"/>
      <om:parameter>
        <om:NamedValue xmlns:om="http://www.opengis.net/om/2.0" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
          <om:name xlink:href="offering"/>
          <om:value xmlns:xs="http://www.w3.org/2001/XMLSchema" xsi:type="xs:string">QR.Master@91401</om:value>
        </om:NamedValue>
      </om:parameter>
      <om:observedProperty xlink:href="QR" xlink:title="Discharge"/>
      <om:featureOfInterest xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="91401" xlink:title="Grey at Dobson"/>
      <om:result>
        <wml2:MeasurementTimeseries gml:id="timeseries.1">
          <wml2:metadata>
            <wml2:MeasurementTimeseriesMetadata>
              <wml2:temporalExtent xlink:href="#phenomenonTime_1"/>
            </wml2:MeasurementTimeseriesMetadata>
          </wml2:metadata>
          <wml2:defaultPointMetadata>
            <wml2:DefaultTVPMeasurementMetadata>
              <wml2:uom code="m^3/s"/>
              <wml2:interpolationType xlink:href="http://www.opengis.net/def/timeseriesType/WaterML/2.0/continuous" xlink:title="Instantaneous"/>
            </wml2:DefaultTVPMeasurementMetadata>
          </wml2:defaultPointMetadata>
          <wml2:point>
            <wml2:MeasurementTVP>
              <wml2:time>2019-07-08T14:55:00.000+02:00</wml2:time>
              <wml2:value>151.515134928844</wml2:value>
            </wml2:MeasurementTVP>
          </wml2:point>
        </wml2:MeasurementTimeseries>
      </om:result>
    </om:OM_Observation>
  </sos:observationData>
  <sos:observationData>
    <om:OM_Observation xmlns:om="http://www.opengis.net/om/2.0" xmlns:gml="http://www.opengis.net/gml/3.2" xmlns:wml2="http://www.opengis.net/waterml/2.0" gml:id="o_2">
      <om:type xlink:href="http://www.opengis.net/def/observationType/OGC-OM/2.0/OM_Measurement"/>
      <om:phenomenonTime>
        <gml:TimeInstant xmlns:gml="http://www.opengis.net/gml/3.2" gml:id="phenomenonTime_2">
          <gml:timePosition>2019-07-08T14:55:00.000+02:00</gml:timePosition>
        </gml:TimeInstant>
      </om:phenomenonTime>
      <om:resultTime xlink:href="#phenomenonTime_2"/>
      <om:procedure xlink:href="Hydrometric_Station"/>
      <om:parameter>
        <om:NamedValue xmlns:om="http://www.opengis.net/om/2.0" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
          <om:name xlink:href="offering"/>
          <om:value xmlns:xs="http://www.w3.org/2001/XMLSchema" xsi:type="xs:string">HG.Master@91401</om:value>
        </om:NamedValue>
      </om:parameter>
      <om:observedProperty xlink:href="HG" xlink:title="Height of Gauge (River Stage)"/>
      <om:featureOfInterest xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="91401" xlink:title="Grey at Dobson"/>
      <om:result>
        <wml2:MeasurementTimeseries gml:id="timeseries.2">
          <wml2:metadata>
            <wml2:MeasurementTimeseriesMetadata>
              <wml2:temporalExtent xlink:href="#phenomenonTime_2"/>
            </wml2:MeasurementTimeseriesMetadata>
          </wml2:metadata>
          <wml2:defaultPointMetadata>
            <wml2:DefaultTVPMeasurementMetadata>
              <wml2:uom code="m"/>
              <wml2:interpolationType xlink:href="http://www.opengis.net/def/timeseriesType/WaterML/2.0/continuous" xlink:title="Instantaneous"/>
            </wml2:DefaultTVPMeasurementMetadata>
          </wml2:defaultPointMetadata>
          <wml2:point>
            <wml2:MeasurementTVP>
              <wml2:time>2019-07-08T14:55:00.000+02:00</wml2:time>
              <wml2:value xsi:nil="true"/>
              <wml2:metadata>
                <wml2:TVPMeasurementMetadata>
                  <wml2:nilReason xlink:href="missing"/>
                </wml2:TVPMeasurementMetadata>
              </wml2:metadata>
            </wml2:MeasurementTVP>
          </wml2:point>
        </wml2:MeasurementTimeseries>
      </om:result>
    </om:OM_Observation>
  </sos:observationData>
</sos:GetObservationResponse>