Add interface for variable coefficients to Collections

CHANGELOG.md

library/Collections/BwdTrans.cpp

library/Collections/CMakeLists.txt

library/Collections/Collection.cpp

library/Collections/Collection.h

library/Collections/Helmholtz.cpp

library/Collections/IProductWRTBase.cpp

library/Collections/IProductWRTDerivBase.cpp

library/Collections/Operator.h

library/Collections/PhysDeriv.cpp

library/Collections/PhysInterp1DScaled.cpp

...y/Demos/MultiRegions/Tests/Helmholtz3D_Prism_Deformed.xml

library/MultiRegions/ContField.cpp

library/MultiRegions/ExpList.cpp

library/StdRegions/StdRegions.hpp