Adds exceptions to acdd variable attribute checks
Adds exceptions to acdd variable attribute checks

 - Exceptions are platform variables and 'crs'.
 - There may be more exceptions we'll need to add in the future

These exceptions were identified in a series of emails asking about the
treatment of QC flags. It was identified that QC flags don't need units
and that we shouldn't be checking the attributes of non-geophysical
variables. The most common non-geophysical variables identified were crs
and platform.

DanielJMaher/compliance-checker,duncombe/compliance-checker,abirger/compliance-checker,lukecampbell/compliance-checker,petejan/compliance-checker,aodn/compliance-checker,ocefpaf/compliance-checker,ioos/compliance-checker