Updates SpecializationTests.
BROKEN: BasicTest.

which wrongly yielded 0 for very small q.

Rearranged formula. At this occasion, switched to series expansion for qR<5e-5.

This closes issue 1294 (provide substantial tests for form factors).
We are now testing:
- whether F(0)=V
- symmetry under reflections and rotations (selectively, not for all shapes)
- specialization (e.g. whether a pyramid with alpha=pi/2 behaves like a prism)
- continuity (comparing F(q) for small q with F(0))
And indeed, some inaccuracies and errors were found (tests outcommented,
tagged "TODO"). See separate issues.

not ready fro merge because severe errors were detected in three form factors.

This will allow us to procompute diameter (= characteristic length scale)
before creating polyhedral faces.

Unit tests now handled by module UnitTests.cmake.
Core tests splitted in several directories, to allow parallel compilation.