ENH: finiteArea: enable gradient caches

Summary

The gradient chaching functionality is enabled for the finite-area framework.

Example usage in system/faSolution:

cache                                                                           
{                                                                               
    grad(h);                                                                    
    grad(Us);                                                                   
}

Risks

• No change in existing input/output.

Metadata

• linux64ClangDPInt32Opt (clang13)
• linux64GccDPInt32Opt
• linux64GccSPDPInt64Debug
• Alltest: No new error/No change in existing output
• Test case: $FOAM_TUTORIALS/finiteArea/liquidFilmFoam/cylinder