Cleaned Cellmodel Interface

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

src/electrophysiology/solvers/LinearImplicitSolver.cpp

src/electrophysiology/solvers/LinearImplicitSolver.hpp

test/cellmodels/MTest.hpp

test/cellmodels/TestTensionModel.cpp