Advance the distribution function, f, discretized into the Lgrid, L, forward in time by a timestep, Tdelta. The off-grid current and next diffusion coefficients, D[m,p]_[old,new] will be used. The number of grid points is set by NL.
This function performs the same calculation as the C-based code, spacepy.lib.solve_cnp. This code is very slow and should only be used when the C code fails to compile.