[feature] human-readable representation of `RPMMetadata.pvm_keys`

[mrossinek]

The RPMMetadata.pvm_keys attribute currently simply stores the index of the PVM in the context of the POVMImplementation which generated the parameters.

It would be nice, if we provided a more human-readable format for inspecting this data.

For example, in the case of (LocallyBiased)ClassicalShadows we could properly associate the indices 0, 1, 2 with their respective measurement basis X, Y, Z (possibly a different order) and make a nicer visual representation when inspecting the pvm_keys.

[mrossinek]

With #26 nearing completion, the possible twirling should be taken into account for such a human-readable representation. Maybe we can do smth along the lines of:

pvm_idx	meaning	visual repr.
0	untwirled X	X
1	untwirled Y	Y
2	untwirled Z	Z
3	twirled X	x
4	twirled Y	y
5	twirled Z	z