[MaL] Replace Gauss-Legendre algorithm with compile-time implementation.

Any weighted sum of type sum(f(x_i)*w_i) can be computed at compile-time
provided constexpr input. This is implemented in Integration.h file in
WeightedSum<Method>::add(f).

The integration method must provide values for x_i and w_i corresponding
to its order. An example implementation is the Gauss-Legendre integration
method up to fourth order.
Add constexpr.