FD coefficients

Second order:

                       deps h - 2 eps        2 eps      deps h + 2 eps
      coeffs = [[a = - --------------, b = - -----, c = --------------]]
                               2               2                2
                            2 h               h              2 h

Fourth order:
                            
               deps h - eps        2 deps h - 4 eps        5 eps
coeffs = [[a = ------------, b = - ----------------, c = - -----, 
                      2                     2                 2
                  12 h                   3 h               2 h
                                         2 deps h + 4 eps        deps h + eps
                                     d = ----------------, e = - ------------]]
                                                  2                     2
                                               3 h                  12 h