Add support for std::vector in function arguments

[vaithak]

Example program which fails currently:

#include "clad/Differentiator/Differentiator.h"
#include <vector>
#include <iostream>

double f (std::vector<double> arr) {
  return arr[0] * arr[1];
}

int main () {
  auto f_dx = clad::differentiate(f, "arr[0]");
  std::vector<double> arr {1,2,3};
  std::cout << f_dx.execute(arr) << std::endl;
  return 0;
}

And similarly for reverse mode.

[wymux]

Where would be the starting point for adding support for vectors?