feat(PhyslibAlpha/QuantumMechanics): product states of a product of finite targets

[naype888-cloud]

hello guys joseph, timeroot, another short one. what this one does?
it adds "FiniteTarget/ProductState.lean" (236 lines), it comes directly from #1699 (operators on one coordinate). the product state (psy * phi)(a,b) = psy a * phi b describes two uncorrelated coordinates, and an observable acting on one coordinate only sees that coordinate.

1.- "prodVec", "norm_prodVec": the product vector, ||psy * phi|| = ||psy|| ||phi||.
2.- "onFst_prodVec", "onSnd_prodVec": (A * 1)(psy * phi) = A psy * phi and (1 * B)(psy * phi) = psy * B phi.
3.- "onFstObservable", "onSndObservable": an observable acting on one coordinate alone.
4.- "variance", "covariance", "centeredGramDefect": on a product state, the mean, variance, covariance and centered Gram defect of a one-coordinate observable are exactly the ones of the factor state.

why? with this one the uncertainty of one chain can be lifted to a product of chains, one axis at a time.

tks a lot, have a nice day guys, im waiting for the review and happy to make any changes¡

[github-actions]

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[jstoobysmith]

Approved - will merge shortly