Removed redundant _3dMat_F.
Removed _3dMatGrad_F (which seemed completely unused since graddp does its own thing anyway). GradDp should use a separate function call instead.
Made some helper functions static.
Removed MRM_Density in favor of the existing routine for asking for material parameters "give".
Removed unnecessary "NonlinearFluidMaterial::giveCharacteristicMatrix".