Add Composition.replace()

[janosh]

Is your feature request related to a problem? Please describe.

Composition does not support elemental substitutions.

Describe the solution you'd like

New method Composition("Fe2O3").replace({"Fe": "Mg"}) similar to Structure.replace_species().

Describe alternatives you've considered

None.

Additional context

Useful when working with data-mined substitution matrices.

[shyuep]

Composition is meant to be immutable. Which is why it inherits Mapping and Hashable and not dict or MutableMapping. Composition objects are probably the one of the most used keys in dictionaries and sets. So in place modifications are not allowed. FYI, the above can be easily done using Composition({(k if k.symbol != "Fe" else "Mn"): v for k, v in comp.items()}).

[shyuep]

Just a note that I have no objection if you want to create a method to do substitutions that return a new composition object. But in-place modifications are not supported.

[janosh]

Probably should have stated explicitly that I didn't want this op to be in-place.