Issues #101: SpatialInertia.__add__ method fix and test

[jcao-bdai]

Fix issue https://github.com/bdaiinstitute/spatialmath-python/issues/101 identified by Loïc Niederhauser (@niederha).

• unit tests are added for basic functionality test;
• it's noted that there are still some methods not fully implemented/tested, will need follow-up;
• this PR includes some formatting change since black formatter has been enabled.

[codecov[bot]]

Codecov Report

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

Comparison is base (a0bb12f) 76.47% compared to head (612712d) 76.84%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #109 +/- ## ========================================== + Coverage 76.47% 76.84% +0.36% ========================================== Files 24 24 Lines 5136 5136 ========================================== + Hits 3928 3947 +19 + Misses 1208 1189 -19 ```

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[niederha]

Thanks! Seems to work, I indeed didn't understand the code right as well, so thanks for commiting and sparing me a costly investigation. While we're on this piece of code I made a minor comment. Otherwise sounds good to me.