Write charge in elementary charge unit (XML writer)

[daico007]

Currently, the XML writer will write out coulomb charge by default. This is partially due to the missing u.Unit("elementary_charge") in the unit default registry (see #792 for related issue). This PR will change the default to write out in elementary_charge, by using the custom registry GMSOUnitRegistry to store the charge unit and write them out to ff xml.

[codecov[bot]]

Codecov Report

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

Comparison is base (c99799a) 92.37% compared to head (f802a41) 92.35%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #794 +/- ## ========================================== - Coverage 92.37% 92.35% -0.03% ========================================== Files 66 66 Lines 6861 6864 +3 ========================================== + Hits 6338 6339 +1 - Misses 523 525 +2 ```

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