Generically support MPS/MPO with multiple site indices and link indices per tensor

[mtfishman]

Not all MPS/MPO functions support MPS/MPO with multiple (or no) site indices and link indices, and often assume there are exactly one site index/link index. An example is that by default, calling:

siteinds(psi)
linkinds(psi)

return Vector{Index}, where in general it would be better if they returned Vector{IndexSet}, allowing for the possibility of zero/more than two site or link indices. This would allow much more general MPS/MPO to be used with built-in ITensor functions (imagine for example Choi matrices with multiple site indices or treating a column or row of a PEPS as an MPS that might get gauged or have gates applied to it).

Supporting this generally could be done fairly easily, but some changes may be breaking like the above change to siteinds/linkinds.