PEP 8 iris.coord_categorisation
ENH: Optimised unit equality special method

This commit results in a 5.4% gain over the _OrderedHashable equality
method utilised for unit comparison.  This implements an interface
through ctypes to UDUNITS ut_compare for efficient unit comparison.
Merge pull request #651 from cpelley/unit_str

ENH: Optimised unit equality special method
Merge pull request #645 from rhattersley/pep8-iris.coord_categorisation

PEP 8 iris.coord_categorisation
Fixup comments
Merge pull request #654 from rhattersley/pep8-iris.coord_categorisation

PEP 8 follow-up: fixup comments in iris.coord_categorisation
ENH: working animation of cubes

This enhancement adds the ability to animate iris cubes along specified
dimensions/coordinates.  This is an experimental feature as the API is
likely prone to change.
review changes
minor change based on review
Modification of feedback from incorrect usage

Changed raised errors to issued warnings to allow user defined plotting
functions to be used.  Also, changed output message for clarification.