Make Orbit propkw equality and hashing consistent

[sylvesterkaczmarek]

Summary

Make Orbit equality and hashing consistent for propagation keyword mappings.

Changes

• Require matching propkw key sets before comparing values.
• Return NotImplemented when comparing an Orbit with an unrelated type.
• Hash propkw entries in sorted-key order so insertion order does not affect identity.
• Canonicalise scalar and array propkw values before hashing, allowing ordinary scalar values such as area=2.0.
• Add regression coverage for asymmetric key sets, reversed comparisons, reordered mappings, and scalar hashing.

Testing

• python3 -m py_compile ssapy/orbit.py tests/test_orbit.py
• git diff --check

The local environment does not have the SSAPy pytest dependency stack installed, so the repository CI will provide the full test run.

Fixes #139

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.39%. Comparing base (a895a5b) to head (b2a03f7).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #140   +/-   ##
=======================================
  Coverage   97.39%   97.39%           
=======================================
  Files          17       17           
  Lines        5820     5829    +9     
=======================================
+ Hits         5668     5677    +9     
  Misses        152      152           

Flag	Coverage Δ
unittests	97.39% <100.00%> (+<0.01%)	⬆️

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Files with missing lines	Coverage Δ
ssapy/orbit.py	98.93% <100.00%> (+0.01%)	⬆️

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[sylvesterkaczmarek]

Thank you for merging this. I really appreciate it, and I’m glad to keep contributing to SSAPy.