coops: Include datum information

[pmav99]

As discussed yesterday in the meeting, it might be a good idea to include datum information in the COOPS metadata/data.

I only had a brief look, but I think that in order to retrieve the datum of each station you need to make an extra HTTP request for each station. E.g. https://api.tidesandcurrents.noaa.gov/mdapi/prod/webapi/stations/1611400/datums.json

That being said, if I understand this correctly, the data we retrieve from COOPS are being requested using the STND datum (i.e. Station Datum) by default which I believe is 0 for all the stations. This might be a bit problematic when you request data from multiple stations because the values will not be comparable across stations.

Another issue, that we might have is what happens when you request historical data. Some stations might be referencing a different epoch, which means that we might need to retrieve the "supersededdatums" instead. E.g. https://api.tidesandcurrents.noaa.gov/mdapi/prod/webapi/stations/2695540/supersededdatums.xml