moved ode_system to coupled_ode_system
Fix stan::math::exp behaviour for NaN (for matrices of doubles)
Using iterators instead (in stan::math::exp)
updating exp to call the unqualified one, which should be stan::agrad::exp
stan::math::exp(Eigen::Matrix) will have an specialized version for double that will check for nan inputs
even simpler and better solution
added NaN test for exp
cleaning up ode code
ode: pushed creation of double versions into coupled_ode_systems
ode: moved num_eqn into coupled_ode_system constructor; changed to N_
ode: added M_ and a size_; added tests for size()
ode: added initial_state() function
ode: made sure each call to the base system only has the size of the original system
ode: renamed variables