Preserve operator structure under contraction

[mtfishman]

Summary

* between an ITensorOperator and another tensor now returns an ITensorOperator whose codomain and domain are composed from the operands, rather than decaying to the bare wrapped state. The domain => codomain pairs of both operands form a graph of maximum degree two (each name is paired at most once per operand), so the result's codomain/domain is found by following each surviving domain name's pairing forward through any contracted shared names. A leg whose chain dead-ends on a contracted index is left dangling, so the product is well defined for any contraction, including a shared dangling link (the c† * c hopping pattern) and a paired index that chains two operators together.

The product is always an ITensorOperator, even when its codomain and domain both come out empty, so the contraction is type-stable regardless of which names happen to contract. apply and apply_dag now contract through state, so they are unaffected by the operator-preserving *.

operator also accepts named ranges (such as Indexes) for its codomain and domain, not just dimension names, so callers can pass the ranges they already have.

[codecov]

Codecov Report

❌ Patch coverage is 96.55172% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 71.60%. Comparing base (705dcc2) to head (b8e01ca).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/itensoroperator.jl	96.55%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #185      +/-   ##
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+ Coverage   71.14%   71.60%   +0.45%     
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  Files          28       28              
  Lines        1428     1451      +23     
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+ Hits         1016     1039      +23     
  Misses        412      412              

Flag	Coverage Δ
docs	15.99% <6.89%> (-0.34%)	⬇️

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