Add an icosahedral sphere mesh class

We can programmatically generate an icosahedral approximation to the
sphere by recursively subdividing the faces of a regular icosahedron
into four triangles.  Do that and add two utility classes.

IcosahedralSphereMesh generates a mesh with specified sphere radius and
refinement level.

UnitIcosahedralSphereMesh generates a unit radius mesh with specified
refinement level.
Test assembly of volume integrals on surface of sphere

Integral of the cell area approximates the surface of the sphere.
Integral of an extruded annulus is xfailed due to lack of support in
FFC.