Add suppress invalid units warning messages context manager
Review actions: func moved to unit.py
Test improvements
Generalise regular interpolation.
Use some Python3 idioms.
Use print function instead of statement.
Use absolute imports.
Add __future__ division on files where it matters.

These are the files where the code expects integer division, and so
needs to explicitly ask for it when using __future__ division.
Merge pull request #1281 from dkillick/units_warnings

Suppress invalid units warnings
optimising
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.