Transition matrix new design

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@KlemKlem's design:

Current state (after the first commit):

I still need to add a few bits, so please don't merge it now.

Though, I think that legend (for direction and polarization) has to be somewhat inside the matrix, not outside. Also, it may be nice to have a similar thing (i.e. inside or near, not separately) with:

• dir and pol order
• polarization basis

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Added dimension labels and swap.

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Hierarchical indices indeed look much nicer.

Still to go:

• legend
• selected entires
• preserving selection with dir<->pol change
• (bigger) basis change

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