Add outputinds/inputinds and outputaxes/inputaxes accessors

[mtfishman]

Summary

Adds outputinds/inputinds and outputaxes/inputaxes, the Index-returning accessors for an operator's codomain and domain spaces (the output space it maps onto and the input space of states it acts on). The inds forms return a Vector and the axes forms a Tuple, mirroring how inds and axes relate for a tensor. The Tuple forms feed directly into constructors like id, randn, and project that take a (codomain, domain) pair of index tuples. The input accessors return the non-dual domain, matching the convention of TensorKit's domain: the conjugate of the input legs as they appear fused in inds, which carry the dual, so a * randn(inputaxes(a)) contracts and id(eltype(a), outputaxes(a), inputaxes(a)) rebuilds an operator of the same shape. A plain tensor is a trivial operator with no pairing, so all four are empty.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.72%. Comparing base (edf6f95) to head (3b45ab6).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #233      +/-   ##
==========================================
+ Coverage   77.66%   77.72%   +0.06%     
==========================================
  Files          30       30              
  Lines        1782     1787       +5     
==========================================
+ Hits         1384     1389       +5     
  Misses        398      398              

Flag	Coverage Δ
docs	26.60% <40.00%> (+0.22%)	⬆️

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