extractClosestPoints() from simplex made more robust to degenerate si…

…mplex (flexible-collision-library#296)

* Expose numerical errors in gjk_libccd-inl.h extractClosestPoints()

If the simplex is degenerate, it can lead to divide-by-zero errors. This
test is drawn from the real world and exposes such a problem - nearest
points are returned as NaN. See
flexible-collision-library#293
for the discussion

* Make extractClosestPoints more robust

For simplex of size n, if the simplex is degenerate, extracts from a
simplex of size n - 1 (to prevent returning NaN).

1. Refactor point extraction into independently callable methods.
2. At each level, add method for downgrading the simplex.
3. Add unit tests on the local methods.
4. Add integration test with motivating example.
5. Update documentation of this implementation.