Add v15 support

[rnestler]

Not yet fully specified. See https://github.com/SpaceApi/schema/milestone/2

Changelog for V15: https://github.com/SpaceApi/schema/blob/master/CHANGELOG.md#v15-unreleased

Open Questions:

• How should we change the builder with regard to multi version support? Currently it is a hard-coded enum, but we probably want to support a list of versions.
• Should we drop v0.13 support once v15 is released and we add support for it?

Features

root

• The linked_spaces field was added (v15: Add linked_spaces, country_code, hint and areas #118)

events

• Clarify that timestamp unit is in seconds

location

• The country_code field was added (v15: Add linked_spaces, country_code, hint and areas #118)
• The areas field was added (v15: Add linked_spaces, country_code, hint and areas #118)
• The hint field was added (v15: Add linked_spaces, country_code, hint and areas #118)
• Make entire location field optional to enable the inclusion of groups without physical location

sensors:

• The power_generation sensor was added
• The carbondioxide sensor was added (v15: Add co2 sensor #123)
• The lastchange field was added to all sensors
• The deprecated unit hPA in barometer.unit was removed, use hPa instead
• The redundant unit mW in power_consumption.unit was removed, use W instead

[rnestler]

@dbrgn I think we should remove support for v0.13 and refactor the multi version support to take an array of compatible versions.