Add SI and CGS unit systems

[adrn]

This removes the classes from the public namespace and adds the realizations si and cgs.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 96.17%. Comparing base (74079d8) to head (ec4bf30). Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #236 +/- ## ========================================== + Coverage 96.12% 96.17% +0.04% ========================================== Files 41 41 Lines 1602 1621 +19 ========================================== + Hits 1540 1559 +19 Misses 62 62 ```

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