Add `firstsiteinds(::MPS)`

To help with generic code for MPS/MPO (i.e. code that works for both), it would help to define firstsiteinds(M::MPS) = siteinds(M).

Also, it may be good to think about the assumption that is often used in MPS functions that MPS have only one site index, since sometimes it can be more natural to have more than one (for example a Choi matrix representing a pure quantum process, or the boundary MPS of a double layer PEPS). For that case, it may be useful to define functions like allsiteinds(::MPS/MPO), that searches for all site-like indices (indices that are not shared with any neighbors in the MPS/MPO) and return them as a vector of IndexSets.