Implement Second-order mixed derivatives and Higher-order mixed derivatives

[alexander-penev]

f(x,y)=x_x + x_y + y*y

df/dx=2_x + y
df/dy=x + 2_y
d2f/dxdy = 1

[vgvassilev]

We support them now, but we have to figure out a 'proper' interface:
Either we do

clad::differentiate(f, 0, 1); // Enumerating the independent variable, i.e first with respect to first arg and then the second

or

clad::differentiate_mixed(f, 0, 1);

or something better.

[vgvassilev]

That is implemented. The interface for the higher order derivative is:

clad::differentiate<N>(f,/*wrt*/ 0);

And for the mixed derivatives:

clad::differentiate(f, /*wrt*/0);
clad::differentiate(f_darg0, /*wrt*/1);