WeatherXds: make STATION_POSITION required, remove antenna_name

[FedeMPouzols]

Resolves #335, Fixes #365.

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[kettenis]

Whether?

[kettenis]

well as?

[kettenis]

The schema changes mean that there no longer is a direct link between antennas and weather stations. So code that wants to do antenna-based corrections based on weather data will need to use some sort of heuristic to find the relevant weaither data. I suppose the default heuristic would be to use the closest station. Potential issue with that approach is that if weather data is only provided for a subset of (VLBI) antennas we'll need to apply some sort of cutoff to make sure we don't use data that is totally irrelevant for an antenna.

Hard to come up with something better though if we want to support both the case where a single weather stations provides information for multiple antennas and the case where multiple weather stations provide information for a single antenna.