Cf-units

Bug fix: https://github.com/SciTools/cf_units/pull/66
Ensure dates are rounded to nearest second when using netcdf4-python. The netcdf4-python behaviour changed between v1.1.1 and 1.2.2.
This release is insulated from that change.

Cross platform bug fix to correctly load udunits2 xml database from `site.cfg`

Documentation:
http://scitools.org.uk/cf_units/index.html

Bug fix to handle calendars when creating a cf_unit from another type of unit

Refactor of cf_units files.

Get cf_units ready for drop-in to Iris.

- Python 3 support