API : tighten validation on pivot in Quiver

only accept {'mid', 'middle', 'tip', 'tail'} instead of
being super permissive.

Closes #3951
PEP8 : comment-related pep8

Fixes 'violations' we don't test for, all white space in comments.
Merge pull request #3955 from tacaswell/quiver_pivot_validate

API : tighten validation on pivot in Quiver