Stop auto-flattening and simplifying TensorProducts

[alexfikl]

This happens for TensorProductShape and TensorProductSpace. Mainly, we should remove

• The simplification in __new__ for "tensor products" with a single object.
• The flattening in __init__.

Motivation: just tried using the code below and it got flattened to 6 PN spaces, which doesn't seem desired.

space = mp.TensorProductSpace((mp.QN(3, 5), mp.QN(1, 5), mp.QN(2, 5)))
assert len(space.bases) == 6

Thoughts?

[inducer]

I agree. If at all, flattening should be something the user can request, not the default.