units_convert and conventions

I have tested this compliance checker against a set of data files created by an application I trust.  Three issues came up:
  1. the conventions attribute is CF-1.5
  2. the units of measure for two coordinate variables are convertible to the canonical unit for the standard name 
  3. the units of measure 'hours' were being picked up as a referenced temporal unit (like 'hours since 1970-01-01') which they are not, due to the way cf.util.units_convertible is implemented

I have made a small number of changes to adapt the checker to handle these cases.

This pull request is not ready to merge as there are test failures.  I would appreciate a little feedback on the logic and on whether there are any known test failures to help me with this change proposal.

many thanks
mark
after testing, that's definitely it.