Compliance reporting: standard name compatibility of units

Following on from #366, a next step in CF Checking capability would be to ensure that (any)where valid standard names are present, the corresponding units are compatible with the standard name (i.e. the units are either those listed as canonical for a given name, else convertible to those according to the UDUNITS database). That is, checking and reporting against the critieria as follows under '3.1 Units' of the Conformance document:

The units of a variable that specifies a standard_name must be physically equivalent to the canonical units given in the standard name table...

where the complication of modifiers could be addressed either together with this, or as a follow-on PR:

...as modified by the standard_name modifier, if there is one, according to Appendix C, and then modified by all the methods listed in order by the cell_methods attribute, if one is present, according to Appendix E.