add groupinds(!) function to merge indices

[starsfordummies]

We need a function to group indices (to basically support some reshaping). Possible use case: if we want to make a simple contraction MPS-MPO => MPS like

| | | | | 
o-o-o-o-o             | | | | | 
| | | | |       =     o=o=o=o=o
o-o-o-o-o

ideally we should be able to do it by something like

psi = rand(Chain, Open, State; n=5, χ=8)
o = rand(Chain, Open, Operator; n=5, χ=8)

Tenet.@reindex!(outputs(psi) => inputs(o))  # I'd prefer if we have a way to make this automatic
opsi=  merge(psi, o)

 for i in inds(psi; set=:physical)
      contract!(opsi, i)
 end

This results in rank 5 tensors, so we need a function to group the resulting virtual indices on matching sites, so that for an MPS of bond dimension chi contracted with an MPO with bond dimension D, we obtain a new MPS with bond dimension chi*D

[mofeing]

Closed by #199