LocalTensors: Do not modify local tensors on slice assignment
LocalTensors: Do not modify local tensors on slice assignment

Also move slice handling from __setitem__ to update().

If multiple consecutive local tensors are update in one go, the bond
dimension should be allowed to change inside the updated region. This is
not possible if we modify the local tensors as before.

The old `lten[:] = v` without shape checks could even perform unintended
actions through broadcasting e.g.  if `lten` has bond dimension two on one end
while `v` has bond dimension one.

dseuss/mpnum,dseuss/mpnum