measure convergence

[luiswirth]

Now after creating poisson{1d,2d} examples, we should measure the convergence of the solution as the mesh width h goes to 0. For this we need to compute relevant error norms like L^2, H^1, and L^oo. For this I need to implement a FeFunction that can be integrated (using quadrature).

[luiswirth]

we are currently able to compute the L^2 norm using trapezoidal quadrature