Mogli/l2norm 2

Adds Class L2Space which provides functions for computing L2 dot products with given quadrature order Q:

• compute L2 norm of functions: std::function e; L2Space<Q> L2; real_t err = L2.norm(e);
• compute RHS of weak form: std::function f; P1Function b; L2Space<Q,P1Function> L2; L2.dot(f,b);