PrismTest: add form factor computation as prism vs as polyhedron
For a thorough test of the conversion from prism to polyhedron, compute the complex form factor f(q) for some off-symmetry q for the original prism, and for the prism as polyhedron, and check whether Re(f) and Im(f) agree within 1e-14*|f|.
To cover cases with and without symmetry Ci, do this test for prisms with trigonal and square base.