Add Composition.replace()

[janosh]

Addresses #2282.

I originally attempted to handle SpeciesLike rather than just str as input, i.e.

elem_map: Dict[SpeciesLike, Union[SpeciesLike, Dict[SpeciesLike, float]]]

instead of the current

elem_map: Dict[str, Union[str, Dict[str, float]]]

but it turned into much more of a hassle with perhaps not much extra value. What's your take?

Checklist

• Code is in the standard Python style. The easiest way to handle this
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• Tests have been added for any new functionality or bug fixes.
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