Unused center pruning

Closes #53

See merge request !48

This reverts commit df726621.

(closes #53 on gitlab).

QBXTargetAssocationFailedException to TargetAssocationFailedException.

Parallel pyfmmlib

See merge request !45

Internals docs improvements

See merge request !47

1) Fix some broken references in the geometry docs.
2) Add target association to the docs.
3) Various minor fixes

(follow up to 721f1ac2e39ec463fda753ea820e21bda953369c in boxtree).

QBXLayerPotentialSource.copy(): Fix _refined_for_global_qbx

Closes #59

See merge request !43

to have a default argument of _not_provided (closes #59 on gitlab).

Without this fix, the copied layer potential source thinks it is not
refined.