Added Support for GetUnits for Python
- Changed FrAdcData.GetUnits to return const STRING& (Fixes #36 (closed))
- Added GetUnits to FrAdcData (fixes #28 (closed))
- Modified return signature of framecpp_init() to work with Buster's python 3 interpreter (fixes #39 (closed))
- Addressed Debian Buster warning messages (Fixes #38 (closed))