Refactor `SimplicialComplex.add_simplex` function

[ffl096]

This pull request makes the implementation of SimplicialComplex.add_simplex a bit simpler and allows to introduce changes more easily in the future. Along the way, an inconsistency when providing a Simplex class with attributes and additional kwargs in different cases was fixed.

[codecov[bot]]

Codecov Report

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

Project coverage is 97.37%. Comparing base (fc47106) to head (13ac77e).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #359 +/- ## ======================================= Coverage 97.37% 97.37% ======================================= Files 37 37 Lines 3543 3543 ======================================= Hits 3450 3450 Misses 93 93 ```

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