allow custom 'units' when saving datetimes to netCDF

[andreas-h]

when manually setting the units attribute of a DataArray containing datetime types, the error

ValueError: Failed hard to prevent overwriting key 'units'

is raised.

It would be good to allow custom units, because currently the units seem to be days since FIRST_TIMESTEP, which leads to different units for different variables within the same Dataset.

[shoyer]

We should raise a better error message. You can indeed actually customize this by setting an appropriate encoding on the DataArray, e.g.,
ds.time.encoding['units'] = 'hours since 1900-01-01T00

[shoyer]

This is now describing on the IO docs page under "encoding"