Add grad rho by rho frozen velocity operator

generate/generate.py

generate/requirements.txt

operators/CMakeLists.txt

operators/grad_rho_by_rho_dot_u/CMakeLists.txt

..._by_rho_dot_u/P2VectorToP1ElementwiseGradRhoByRhoDotU.cpp

..._by_rho_dot_u/P2VectorToP1ElementwiseGradRhoByRhoDotU.hpp

...t_u/P2VectorToP1ElementwiseGradRhoByRhoDotUAnnulusMap.cpp

...t_u/P2VectorToP1ElementwiseGradRhoByRhoDotUAnnulusMap.hpp

...torToP1ElementwiseGradRhoByRhoDotUIcosahedralShellMap.cpp

...torToP1ElementwiseGradRhoByRhoDotUIcosahedralShellMap.hpp

...1ElementwiseGradRhoByRhoDotUAnnulusMap_apply_macro_2D.cpp

...ementwiseGradRhoByRhoDotUAnnulusMap_toMatrix_macro_2D.cpp

...iseGradRhoByRhoDotUIcosahedralShellMap_apply_macro_3D.cpp

...GradRhoByRhoDotUIcosahedralShellMap_toMatrix_macro_3D.cpp

...2VectorToP1ElementwiseGradRhoByRhoDotU_apply_macro_2D.cpp

...2VectorToP1ElementwiseGradRhoByRhoDotU_apply_macro_3D.cpp

...ctorToP1ElementwiseGradRhoByRhoDotU_toMatrix_macro_2D.cpp

...ctorToP1ElementwiseGradRhoByRhoDotU_toMatrix_macro_3D.cpp