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).