Use xarray dataset as internal data container

[jsmariegaard]

Changing the internal data structure of observations from pd.DataFrame to xr.Dataset.

Some considerations:

• When should na values be removed? (and should we keep track na-values? before this PR it was dependent on the input type i.e. different functionality whether you constructed from df or dfs0)
• Can all "matching" be extracted from _comparison and moved to matching.py? Such that Comparer can only be initialized with an already-matched dataset
• TrackModelResult is now almost equal to TrackObservation and PointModelResult is almost equal to PointObservation - should we rather make a TrackTimeseries and a PointTimeseries and have most functionality in there?

Objectives of this PR:

• Consistent .data throughout modelskill
• Make it easy to match modelresults and observations (as everything is now xr.Dataset)
• Make it easy to add aux items to observation and modelresults
• Make it easy to add attrs container to observation and modelresults

[jsmariegaard]

Much more to be done, but maybe good to merge soon to ease collaboration and minimize merge conflicts

[ecomodeller]

The name of this function doesn't reveal that it also remove gaps, also not the docstring. Should gap removal be part of this function.

[ecomodeller]

I tried to modify a few lines like this

ds[name].attrs["quantity"] = quantity.to_dict()

to

ds[name].attrs["long_name"] = quantity.name
ds[name].attrs["units"] = quantity.unit

To be able to persist the dataset as NetCDF and be compatible with CF convention making easier to plot the data with xarray, but somehow managed to change more than necessary, making a lot of tests to fail. ☹️

So I deleted those changes and only committed the tests that saves the data as NetCDF.

[ecomodeller]

Plotting the xarray dataarray uses long_name and units

[ecomodeller]

x, y in track data are now coordinates and not data variables.

[jsmariegaard]

x, y in track data are now coordinates and not data variables.

I was wondering about this. Thanks