Feature/new fields query parameter

[SpeckiJ]

Low-Level implementation of 52North/sensorweb-server-helgoland#169.

Elements are only added to the OutputObject if they are requested (or field parameter is missing).

Also contains smaller mvn checkstyle improvements.

TODO: Documentation (if low-level approach is approved)

[SpeckiJ]

Requires ridoo/series-rest-api#19

[ridoo]

@SpeckiJ could you go for a more streamlined version? Have a look at shutgun surgery to understand why.

What about adding

protected AbstractOutput(IoParameters parameters) {
    this.parameters = parameters;
}

protected boolean isSelectedOutput(String field) {
  return parameters == null || parameters.getFields().contains(field);
}

Not sure if a DbQuery.getParameters() is available wherever an AbstractOutput is created. However, It is worth to check, if you can easily make it available then. The aim is to reuse parameters within the output setters to decide to return null or the actual value (in case it has been requested by the client). For example:

public void setHref(String href) {
    this.href = isSelectedOutput(HREF) // HREF to be added
            ? href
            : null;
}

For sure this would require to adjust each setter. But there are advantages:

1. It keeps selection within the actual POJOs (rather than business classes of different types)
2. It is separated from SPI implementation
3. As output selection is done within setter pattern it can be handled (and even more separated) later via AOP pointcuts

If you have further questions on this, please let me know.