Make `apply` with single-site gate not change orthogonality

For apply with a single-site gate, i.e. apply(op("Sz", s, j), psi), currently it changes the orthogonality center with orthogonalize!(psi, j). However, for single-site gates this isn't necessary, since no truncation is needed and the orthogonality center can be changed later as needed.

Additionally, we could add a check in apply to see if a single-site gate is (approximately) unitary, which would be cheap. For multi-site gates you could do a similar check, but in general you want to truncate so the orthogonality center needs to be changed anyway. We could provide a user flag to specify the threshold that is desired for how approximately unitary a gate needs to be to be considered unitary.