Efficient implementation of `expect` for `AbstractTTN`

[b-kloss]

This PR addresses #128 and adds an implementation of expect that should allow evaluation of local observables at all sites in a TTN at linear cost in the number of vertices in the TTN.

Basic tests have been added:

• comparing with ITensors.expect evaluated on the output of random_mps converted to ITensors.MPS, as well as verifying the Sz
• expectation value for a product-TTN in the computational basis for a comb_tree.

Open issues:

• performance testing should be added at some point / optimality of implementation is not verified
• Currently does not support multiple site-indices per vertex. This could probably easily be addressed similar to the way this is handled in the svd_ttn for converting OpSum to TTN.

[b-kloss]

I'll fix keyword argument signature that makes the solver_tests fail.

[b-kloss]

If tests pass this is ready for review.

[mtfishman]

Looks good, thanks!

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (56fcb93) 72.90% compared to head (36386b9) 72.73%. Report is 1 commits behind head on main.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #129 +/- ## ========================================== - Coverage 72.90% 72.73% -0.17% ========================================== Files 71 68 -3 Lines 4097 3947 -150 ========================================== - Hits 2987 2871 -116 + Misses 1110 1076 -34 ```

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