Add clearer indication for results of differentiation of functions for which no derivative in custom_derivatives exist

[efremale]

Currently, if differentiation process meets a function which has no defined derivative in custom_derivatives namespace, its derivative is set to be 0.

Example:

double f(double x) {
  printf("%d", x);
  return x;
}

->

double f_darg0(double x) {
  0;
  printf("%d", x);
  return 1;
}

Since there exist no custom_derivatives::printf_darg0, we set the derivative of printf to be zero.
In principle, we could detect it and return null instead, but this would break cases when printf is used in expressions, e.g. int i = x + printf(...); -> int _d_i = _d_x + 0;. This is an obscure case, but it makes sense to use 0 in such expressions (say, if it is not printf, but some non-differentiable math function, we cannot determine that).

Should we indicate more clearly where does this 0 come from? E.g.

double f_darg0(double x) {
  int printf_darg0_non_differentiable = 0;
  printf_darg0_non_differentiable;
  printf("%d", x);
  return 1;
}

[PetroZarytskyi]

Currently, when Clad fails to generate a derivative or find a custom one, it attempts to use numerical differentiation. And then a warning is produced, which either says that num diff was used or that the function was not differentiated at all. I believe the issue is fixed now.