Wrong dtype of return in date2num

[zklaus]

📚 Documentation

The documentation for cf_units.Unit.date2num and cf_units.date2num stipulates that both functions return a float, or numpy.ndarray of floats. However, since the return value comes directly from cftime.date2num, this will regularly return "an array of integers" since cftime version 1.4.0 (see the changelog).

[pp-mo]

Re-opened

I just realised that in addition to Unit.date2num(dates), we also have a cf_units.date2num(dates, unit, calendar),
which is basically a different package for the same behaviours.

So unfortunately I think that docstring needs updating too.

and... we really need a whatsnew entry for these changes, too.
Sorry @ukmo-ccbunney I was so enthusiastic about #467 that I seem to have failed to review carefully !

[ukmo-ccbunney]

Fixed by #483