Updates for V2 -> V3 transition

[mtfishman]

Here are some things we should make sure to emphasize in the transition guide/changelog:

• Changed the prime level convention for the output of nmultMPO and applyMPO.
• Removed exactApplyMPO and fitApplyMPO interface in favor of applyMPO.
• Instead of mapprime, use replaceTags (maybe we will add mapPrime back to ease the transition).
• ITensor functions assume MPS and MPO have certain index topology conventions (neighboring tensors have one shared link).