Add enthalpy_units optional argument to Reactant in Python API

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Summary

Add explicit enthalpy_units support to the Python cea.Reactant API with a backward-compatible transition path.
Current implicit behavior is preserved for now (enthalpy defaults to J/kg), but omission now emits a deprecation-style FutureWarning and documentation/examples are updated to use explicit units. Thanks to @minh-dng for flagging the issue. Partial fix for #65.

Changes

• Python API (source/bind/python/CEA.pyx)

  • Added enthalpy_units argument to Reactant(...).
  • Added normalization/validation for enthalpy units (case-insensitive, whitespace-tolerant).
  • Supported units:
    • Weight-based: J/kg, kJ/kg, cal/kg, kcal/kg
    • Molar-based: J/mol, kJ/mol, cal/mol, kcal/mol
    • /mol and /mole spellings accepted and normalized.
  • If enthalpy is provided and enthalpy_units is omitted:
    • preserve legacy default behavior (J/kg)
    • emit FutureWarning recommending explicit enthalpy_units.
  • Enforced molecular-weight requirement only for weight-based enthalpy units (*/kg).
  • Updated Mixture reactant bridging logic to use per-reactant enthalpy units and conversion path:
    • weight-based inputs convert with molecular weight
    • core-facing units remain molar (*/mole).

• Docs and examples

  • Updated:
    • docs/source/interfaces/python_api.rst
    • docs/source/examples/equilibrium/example5.rst
    • source/bind/python/cea/samples/rp1311/example5.py
  • All updated examples now pass explicit enthalpy_units.

• Tests (source/bind/python/tests/test_rp1311_samples.py)

  • Updated Example 5 custom-reactant test to explicit enthalpy_units.
  • Added targeted coverage for:
    • omitted enthalpy_units warning + behavior preservation
    • explicit molar units without MW requirement
    • explicit weight units with MW requirement
    • weight-without-MW error path
    • accepted unit spellings across required set
    • molar/weight equivalence for same physical enthalpy input

Testing

• ctest -R cea_core_test -V
• python -m pytest

Compatibility / Numerical behavior

• No expected changes to numerical results
• Expected changes (explain and provide validation)

Expected behavior changes are API-facing (validation/warnings):

• New FutureWarning when Reactant.enthalpy is given without enthalpy_units.
• New ValueError when weight-based enthalpy units are used without molecular_weight.
• Existing omitted-units numerics are intentionally preserved in this PR (legacy default remains J/kg).