ENH: Streamline and refactor solvers

Examples/Shaders/main.cpp

Solvers/CMakeLists.txt

Solvers/ConjugateGradient.cpp

Solvers/ConjugateGradient.h

Solvers/DirectLinearSolver.h

Solvers/InexactNewton.cpp

Solvers/InexactNewton.h

Solvers/NonLinearSolver.cpp

Solvers/SystemOfEquations.h

Solvers/UnitTests/SolversSpec.cpp.in

TimeIntegrators/BackwarEuler.cpp

TimeIntegrators/BackwarEuler.h

TimeIntegrators/ForwardEuler.cpp

TimeIntegrators/ForwardEuler.h

TimeIntegrators/OdeSystem.cpp

TimeIntegrators/OdeSystem.h

TimeIntegrators/TimeIntegrator.cpp

TimeIntegrators/TimeIntegrator.h

TimeIntegrators/UnitTests/BackwardEulerSpec.cpp

TimeIntegrators/UnitTests/ForwardEulerSpec.cpp