Better handling of non-standard ions when loading from IMAS

I've been supplied with an IDS that has "DT" as a main ion name. This is incompatible with the TORAX loader, and `plasma_composition_from_IMAS` raises an error. How should I load this, or find an alternative route that still lets me load this IDS?? Do we need to add functionality to better support when IDSs have unrecognized ions?

@MateoBell

Start by inspecting the plasma_composition_from_IMAS entry point and reproduce the error with an IDS whose main ion is named "DT". Trace how the TORAX loader handles unrecognized ions, then determine and implement the intended loading behavior or document an alternative route for such IDSs.

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