Add: include enumeration class CaloricGasModel to simplify caloric gas model definition

[AlbertoCuadra]

This PR introduces the CaloricGasModel enumeration to define the caloric gas model explicitly (perfect, thermallyPerfect, imperfect). This replaces the previous dual-flag mechanism (FLAG_TCHEM_FROZEN, FLAG_FROZEN) and avoids invalid combinations that could occur silently.

Backward Compatibility

• Legacy flags are still accepted.
• CaloricGasModel.fromFlag() maps them safely to enum values.

Example

caloricGasModel = CaloricGasModel.perfect;

if caloricGasModel.isPerfect()
    % configure perfect gas behavior...
end