Switch tropical_in_new ordering to omeco

[Morningliumengkun]

Summary

• switch tropical contraction ordering to rely on omeco heuristics only
• update docs/README/examples/tests for omeco-based usage and imports
• add validation improvements and a new evidence parsing test

Test plan

• pytest tropical_in_new/tests (requires omeco + Rust/Cargo)

Fixes #56

[GiggleLiu]

● PR #58 has nothing new compared to PR #51. It's actually a subset — PR #51 contains everything in PR #58, plus the review comment fixes.

The diff shows PR #51 has these additional changes that PR #58 lacks:

1. contraction.py: The einsum-with-elimination refactor (using elim_vars directly in einsum instead of a separate ReduceNode)
2. utils.py: The assert statements for internal consistency
3. tests/: All the additional tests (contraction edge cases, 3-variable MPE, partial order, primitives error paths, utils validation errors)

PR #58's commits ("Add PyTorch BP docs", "Ignore Python cache files", etc.) appear in its commit history but don't change any files beyond what's already
in main — they were already merged via earlier PRs.

In short: PR #58's merge was a no-op since PR #51 (merged 2 seconds earlier) already included all the same content plus more.