Feature/volume

Introduces a helper function to compute the volume of convex polytope geometries. For common convex polytope geometry types there are efficient helpers. A generic method uses the volume() function of the geometry as fallback if a specialized volume function is not implemented (although we cannot guarantee that that method actually returns a good volume approximation.

The helper is called convexPolytopeVolume since it only works correctly for convex polytopes. There is also a helper that uses integration on the reference element to determine the volume for nonlinearly transformed geometry types. For box we only supported and continue to support convex polytopes.