How to write the Nim version of a C function pointer and pass it as argument to C function

[PMunch]

1\. No Looking at the code that `double** jac` should probably be `ptr 
UncheckedArray[ptr UncheckedArray[cdouble]]`.

  2. With the double array in Nim you would assign values to the matrix 
elements the same way as in C, with double square brackets.