added C and Python support for custom reactant data

[markleader]

Summary

Add custom-reactant support to the C and Python interfaces so RP-1311 Example 5 style workflows (including reactants not present in thermo.lib, e.g. CHOS-Binder) are available outside the legacy main interface, while keeping existing Mixture([...]) behavior backward compatible.

Changes

• C API:
  • Added additive constructors in source/bind/c/cea.h:
    • cea_mixture_create_products_from_input_reactants(...)
    • cea_mixture_create_products_from_input_reactants_w_ions(...)
• Fortran C shim (source/bind/c/bindc.F90):
  • Implemented the new product-constructor APIs.
  • Added shared parser helper for cea_reactant_input -> ReactantInput to remove duplicated parsing/validation logic.
• Python bindings:
  • Extended source/bind/python/cea_def.pxd with cea_reactant_input and new function declarations.
  • Added cea.Reactant in source/bind/python/CEA.pyx.
  • Updated Mixture(...) to accept str, Reactant, and mixed lists.
  • Added dispatch to input-reactant creation paths (including product generation from input reactants).
  • Enforced SI-only Python Reactant inputs:
    • temperature in K
    • enthalpy in J/kg (converted internally for core input)
• Examples:
  • Added C sample: source/bind/c/samples/rp1311_example5.c
  • Implemented Python sample: source/bind/python/cea/samples/rp1311/example5.py
  • Registered C sample in source/bind/c/CMakeLists.txt as cea_bindc_rp1311_ex5
• Tests/docs:
  • Added RP-1311 example 5 Python regression coverage in source/bind/python/tests/test_rp1311_samples.py
  • Removed bytes-mixture test in source/bind/python/tests/test_strings.py (bytes inputs no longer supported)
  • Updated Python API docs in docs/source/interfaces/python_api.rst to document cea.Reactant and SI expectations.

Testing

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

Compatibility / Numerical behavior

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