Add utility to compute "h orthogonal"

[fdrmrc]

A good penalty apparently is $\sigma(x)=10\frac{(p+1)(p+d)}{h_f^{\perp}}$.

The goal of this PR is to enable the computation of $h_f^{\perp}$. To this end, on a given face $f$ of a polygon $P$, I shoot a ray from the midpoint of $f$ and compute the point, say $q$, intersected by such a ray. Then, the distance of $q$ from the source is computed. Internally I am using CGAL, hence the implementation is guarded by DEAL_II_WITH_CGAL.

I've added a minimal test with some polygons.

Depends on #54 #56 , the relevant commit is https://github.com/fdrmrc/Polydeal/pull/57/commits/baecbf29899afe4be0e7f178e1ce1d75e58c683b

Before merging, the following tasks should be done:

• [ ] extend to 3D
• [x] test it in a solver

[fdrmrc]

I'll do the 3D in a follow up PR. The penalty has been already tested, so I'll merge.