Fixed RHSUpdate usage

src/cellmodels/MBeelerReuter.cpp

src/cellmodels/MBeelerReuter.hpp

src/cellmodels/MCellModel.cpp

src/cellmodels/MCellModel.hpp

src/cellmodels/MFitzHughNagumo.cpp

src/cellmodels/MFitzHughNagumo.hpp

src/cellmodels/MRossi.cpp

src/cellmodels/MRossi.hpp

src/cellmodels/MTenTusscher.cpp

src/cellmodels/MTenTusscher.hpp

src/cellmodels/solvers/MElphySolver.cpp

test/cellmodels/MTest.hpp