Add suppress invalid units warning messages context manager
Review actions: func moved to unit.py
Test improvements
Merge pull request #1281 from dkillick/units_warnings

Suppress invalid units warnings
Remove unused 'warnings' imports.
Remove unused 're' imports.
Remove unused 'collections' imports.
Remove unused 'datetime' imports.
Remove unused 'operator' imports.
Remove unused 'os' imports.
Remove unused 'sys' imports.
Remove unused 'math' imports.
Remove unused 'mock' imports.
Remove unused 'OrderedDict' import.
Remove unused NumPy imports.
Remove unused 'biggus' imports.
Remove unused 'Image' import.
Remove unused Cartopy import.
Remove unused 'gribapi' imports.
Remove unused 'iris' imports.