serialization as netcdf throws an exception

[pmav99]

I am not sure if xarray-selafin, wants to support exporting selafin files as netcdfs, but the way the xarray attributes are currently being utilized is not compatible with netcdf. For example:

ds = xr.open_dataset("tests/data/r2d_tidal_flats.slf")
ds.to_netcdf("/tmp/out.nc")

results in:

TypeError: Invalid value for attr 'variables': {'U': ('VELOCITY U', 'M/S'), 'V': ('VELOCITY V', 'M/S'), 'H': ('WATER DEPTH', 'M'), 'S': ('FREE SURFACE', 'M'), 'B': ('BOTTOM', 'M')}. For serialization to netCDF files, its value must be of one of the following types: str, Number, ndarray, number, list, tuple