Add `sasdata.quantities.units` to `.gitignore`

[krzywon]

The unit conversion and unit handling system in the refactoring branch (sasdata.quantities.units) is a generated file, but also lives in the repo. When changes are made to the file that generates this file, but need to be committed, even though the changes in the base file nearly matches the changes in the generated file. Ideally, units.py should be added to .gitginore and removed from the repo entirely.

My concern with this is someone editing units.py instead of the base _units.py and then trying to generate the file which will destroy all of the changes.