Cf-units

cf_units was renamed to cf-units for broader consistency with the scientific python naming practices

Fix source tar-ball distribution, which was missing necessary cython files.

Technically, this release should have no breaking changes between v1.x and v2.x.

However, the major change in this release is that udunits is now wrapped via cython, not ctypes. This is in response to costly import times being experienced on distributed filesystems such as Lustre (where gcc was being called to identify the libudunits.so path).

This means that the cf_units/site.cfg is now no longer used to identify the location of libudunits, and standard LD library path definitions may be used instead.

* Use succinct unit string for str (79)
* Handle reference time units of months & years appropriately (72)

Fix for num2date/date2num allowing support for multi-dimensional arrays.