Allow DGP elements

[fdrmrc]

Related to #127. DGP elements are now supported. They turn out to be convenient under p-refinement. See the next example, where the polynomial degree increases for a given unstructured polytopal mesh.

[fdrmrc]

Some more plots. Each dot in the plot is associated to a polynomial degree $p$. Here we go from $p=1$ to $p=6$ and check the $L^2$ (first figure) and semi-H1 norm (second figure) of the error. For a fixed number of DoFs, using $P_p$ elements always gives better accuracy than $Q_p$ elements. FYI @luca-heltai @andreacangiani