Add support for std::get and template arguments in pushforwards/pullbacks

[gojakuch]

currently, if there's a function accepting a template argument that cannot be resolved automatically by inferring it from its arguments (like std::get for example), it's not possible to use the automatically generated pushforward/pullback for it:

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

template<int N>
double take(double x, double y) {
    return 0;
}
template<>
double take<0>(double x, double y) {
    return x;
}
template<>
double take<1>(double x, double y) {
    return y;
}

double fn(double u) {
    double a = take<0>(u, 2*u);
    double b = take<1>(u, 2*u);
    return a+b; // = 3*u
}

int main() {
    auto df = clad::differentiate(fn, "u");
    std::cout << "f(3)=" << fn(3.0);
    std::cout << "; df(3)=" << df.execute(3.0) << '\n';
}

expected output:
f(3)=9; df(3)=3
output:
f(3)=9; df(3)=2

while Clad does create two different pushforwards for these specialisations, it only calls one of them, failing to distinguish between those.

std::get when used as std::get<10>(my_tuple), for instance, would cause this too, which is why it's important to solve this issue to fully support STL or even tuples.

this must be solved in both fwd and rvs modes in a way that also allows custom derivatives to be provided for these functions.