Units/Windows kludge: camelcase Pascal

The symbol `PASCAL` is defined by default on Windows
and is used **heavily**, meaning likely any workaround
that we come up with will break *somewhere*.

Rather than batter further, this commit makes
`PASCAL` the one exception to the rule of uppercase
unit enumeration values.

/cc @rleigh-dundee 

Check the status of https://ci.openmicroscopy.org/view/Failing/job/OMERO-5.1-merge-cpp-win/ for merging.