Implement canonize_site! for MPO

[jofrevalles]

Since PR #232, we have defined canonize for AbstractMPO struct. I believe this should be AbstractMPS, since for now we don't have canonize_site! (the inner function called inside canonize). @mofeing

See:

julia> using Tenet

julia> mpo = rand(MPO, n=10, maxdim=10)
MPO (inputs=10, outputs=10)

julia> canonize!(mpo)
ERROR: MethodError: no method matching canonize_site!(::MPO, ::Site{1}; direction::Symbol, method::Symbol)
The function `canonize_site!` exists, but no method is defined for this combination of argument types.

Closest candidates are:
  canonize_site!(::MPS, ::Site; direction, method)
   @ Tenet ~/git/Tenet.jl/src/MPS.jl:501

Stacktrace:
 [1] canonize!(ψ::MPO)
   @ Tenet ~/git/Tenet.jl/src/MPS.jl:545
 [2] top-level scope
   @ REPL[4]:1

[mofeing]

In principle, the only thing you need to do for canonize! to work with MPO is to implement canonize_site! for MPO.