Re-create SMIRNOFF force field from tensor force field

[mattwthompson]

Closes #146

Tests

• Perturb parameter in a section, compare result
  • vdW
  • Bonds
  • Angles
  • Proper torsions
  • Improper torsions
  • Electrostatics skipping this
  • Virtual sites
• Un-modified force field is returned un-modified
• Double round-trip through conversions

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.37%. Comparing base (0b3a51a) to head (debfbf0).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #166      +/-   ##
==========================================
+ Coverage   81.29%   81.37%   +0.08%     
==========================================
  Files          55       56       +1     
  Lines        5041     5064      +23     
==========================================
+ Hits         4098     4121      +23     
  Misses        943      943              

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[Copilot]

Copilot review overview

🟢 Approval recommended

The core implementation and tests look sound, with only minor cleanup issues (unused import, debug print, typos) remaining.

Review effort: Lite
Findings: 3

Open (3)

• Fix comment typo: “in on” to “in one” · New
• Remove debug print from test · New
• Remove unused openff.interchange import · New

What changed in this PR

Adds a converter that reconstructs a SMIRNOFF openff.toolkit.ForceField from a tyff.TensorForceField, using an existing ForceField as the template (addressing #146 and enabling round-trips between tensor and SMIRNOFF representations).

Changes:

• Added convert_tensor_force_field(...) to write tensor parameter values back onto a deepcopy of an input SMIRNOFF ForceField (skipping electrostatics and not yet supporting v-sites).
• Exported the new converter from tyff.converters.
• Added tests covering no-op conversion, double round-trip stability, and parameter perturbation round-trips (with an explicit v-site unsupported case).

File	Description
tyff/​converters/​openff/​_tensors.py	Implements tensor → SMIRNOFF conversion by applying tensor parameters onto a template ForceField.
tyff/​converters/​__init__.py	Exposes convert_tensor_force_field at the tyff.converters API surface.
tyff/​_tests/​convertors/​openff/​test_tensors.py	Adds test coverage for round-trips, perturbations, and v-site unsupported behavior.

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