Element composition dict with str and mixed keys

[lmseidler]

• added the capability to use composition dicts with string keys instead of atomic number keys, as well as mixed keys
• added test cases for dict[int, tuple[int, int]], dict[str, tuple[int, int]], dict[str | int, tuple[int, int]]
• added check for validity of provided atomic numbers in composition dict (check, if atomic number in PSE table)
• side-effect is that element composition is effectively a copy of the input argument, protecting the attribute from possibly unwanted changes due to passing by reference when using a dict as input

[marcelmbn]

Looks great! Thanks for contributing!

Before merging:

1. Have you checked if pytest -vv --optional succeeds as well? (Contains all tests that depend on external non-Python dependencies.)
2. Can you update the Python API description in README.md and the corresponding lines/descriptions in the sample mindlessgen.toml (only if applicable)?
3. Can you add a line to CHANGELOG.md, describing your changes?

[marcelmbn]

On my machine, all tests (including --optional) pass on branch lmseidler:main.
So, just points 2 and 3 in the previous comment are left. 🙂

[marcelmbn]

Great, thanks for your contribution!