Generalize `inner(::MPO, ::MPS, ::MPO, ::MPS)` for arbitrary tags and prime levels

[leburgel]

Description

Generalizes inner(::MPO, ::MPS, ::MPO, ::MPS) to no longer rely on any tag or prime level conventions. It is now only required that for inner(A::MPO, x::MPS, B::MPO, y::MPS):

• A and x share site indices.
• B and y share site indices.
• A * x and B * y share site indices.

Fixes #499

How Has This Been Tested?

Extended the "inner <By|A|x>" unit test in test/mpo.jl.

Checklist:

• [x] My code follows the style guidelines of this project. Please run using JuliaFormatter; format(".") in the base directory of the repository (~/.julia/dev/ITensors) to format your code according to our style guidelines.
• [x] I have performed a self-review of my own code.
• [x] I have commented my code, particularly in hard-to-understand areas.
• [x] I have added tests that verify the behavior of the changes I made.
• [ ] I have made corresponding changes to the documentation.
• [ ] My changes generate no new warnings.
• [ ] Any dependent changes have been merged and published in downstream modules.

[mtfishman]

Thanks! Once tests pass I'll merge this.

[codecov-commenter]

Codecov Report

Merging #995 (888afcf) into main (8fc77d1) will decrease coverage by 0.00%. The diff coverage is 100.00%.

:exclamation: Current head 888afcf differs from pull request most recent head 42132c7. Consider uploading reports for the commit 42132c7 to get more accurate results

@@            Coverage Diff             @@
##             main     #995      +/-   ##
==========================================
- Coverage   64.90%   64.90%   -0.01%     
==========================================
  Files          97       97              
  Lines       10173    10169       -4     
==========================================
- Hits         6603     6600       -3     
+ Misses       3570     3569       -1     

Impacted Files	Coverage Δ
src/mps/mpo.jl	84.71% <100.00%> (-0.12%)	:arrow_down:
src/ITensorChainRules/ITensorChainRules.jl	84.61% <0.00%> (-7.70%)	:arrow_down:
src/tensor_operations/matrix_algebra.jl	93.02% <0.00%> (-2.33%)	:arrow_down:
src/physics/lattices.jl	95.65% <0.00%> (-2.18%)	:arrow_down:
src/mps/sweeps.jl	90.69% <0.00%> (-1.17%)	:arrow_down:
src/tagset.jl	86.95% <0.00%> (-1.09%)	:arrow_down:
src/ITensorChainRules/mps/abstractmps.jl	88.17% <0.00%> (-1.08%)	:arrow_down:
src/qn/qn.jl	88.51% <0.00%> (-0.86%)	:arrow_down:
src/LazyApply/LazyApply.jl	80.10% <0.00%> (-0.52%)	:arrow_down:
src/physics/sitetype.jl	90.78% <0.00%> (-0.44%)	:arrow_down:
... and 3 more

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