Js reactions

Try to merge changes that fix computation of ractions
This class should have more specific name like: LinearODESolver, with clearly specified interface, there should be matrix na RHS comming in and then there should be some think like update_solution to make one step. All data specific to particular method should be private. It's a question if it is better having vector of unknowns as internal variable or just passed in as parameter of update_solution. In future I imagine this as special case of much broader concept of class templates implementing various ODE solvers for various kind of equations (linear, non-linear, with different kind of matrices, etc.).