mallory — system of differential equations

Via Numerical.rk4, which was already a vector-state (system) solver -- extending /ode's single-equation solver to two coupled equations dx/dt = f(x,y,t), dy/dt = g(x,y,t) only needed new glue code, not a new numerical method. v1 is fixed at 2 equations/2 variables (the same scope cut SystemSolverPanel made for algebraic systems), with one trajectory from a single initial condition overlaid on a direction field sampled at t0 -- no multi-trajectory overlay, no animation.