Allow file missing data indicator values to be stored on NetCDFArray instances

[davidhassell]

It can be useful to applications to know the missing data indicator without opening the netCDF file. This is the case for active storage reductions currently being implemented in cf-python.

It is almost always the case that the netCDF file is open and available when the NetCDFArray instance is created, so it is easy for these to be stored on the object if desired, e.g.

>>> n = NetCDFArray(
...     'file.nc', 'ncvar', missing_values={"missing_value": -999.0, "valid_range": (-111, 222)}
... )