Speed up coordgen, especially for very slow molecules

[d-b-w]

Improve coordgen performance: #39

When detecting clashes, do a rough binning of atoms. If both atoms for one bond are above both atoms of the other bond, they can't be a clash. This saves clash calculations for things that are obviously not clashes. It has an impact on big structures that are partially templated. (overall 15% speedup, but 40% improvement for the worst molecules in the test case.

coordgen is still slower than RDkit's native coordinate generation, though.

Also adds a simple test that the coordinates generated don't have crazy bond lengths. This would have caught my earlier misunderstanding.

[d-b-w]

oh, bummer. It looks like the sketcherMinimizerClashInteraction objects are reused, sometimes not even with the same atoms. Caching the distance in the sketcherMinimizerClashInteraction leads to incorrect results, though fewer problems than I would have expected for the cost.

I'll back out that part of the change.

[d-b-w]

I pulled out the refactorings I was doing to try to understand things into a separate set of commits. This is now only the performance improvement, a test, and a change to remove some unused headers to speed up my rebuilds.

[ricrogz]

@d-b-w, the CI builds failed, apparently, because of an unused variable (travis) and some constexprs (appveyor).

[d-b-w]

Thanks, @ricrogz . I see 'em.