Unitpy

Expand prefix to all units.
Add custom errors.

Fix bug with units that have offset

What's Changed
* fix bug with mass by dylanwal in https://github.com/dylanwal/unitpy/pull/7
**Full Changelog**: https://github.com/dylanwal/unitpy/compare/v0.0.12...v0.0.16

Add numpy support

Fix bugs.
Improve string formatting.
Add support for .env

Add datetime conversion and pickling