Coil limits as attribute for Coil

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@chrismarsden7 and I have been discussing about the best way to find solutions within coil limits.

I'd propose to add min and max current per turn as an attribute for the base Coil type, which can then be used more generally such as in optimisation routines.

I've got some code working along these lines — can start to work on this if there's no objections to this approach.