Add Newton-Raphson nonlinear solver

DO NOT MERGE THIS PR! It should be rebased after #189 is merged. But you can start reviewing MathLib/Nonlinear/NewtonRaphson.h, NewtonRaphson-impl.h, and  Tests/MathLib/TestNonlinearNewton.cpp (or de7720c, 8220c4c, 5c9671f).