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.

QBXLayerPotentialSource.copy(): Fix _refined_for_global_qbx

Closes #59

See merge request !43

(follow up to 721f1ac2e39ec463fda753ea820e21bda953369c in boxtree).

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

Internals docs improvements

See merge request !47

Parallel pyfmmlib

See merge request !45