Separate kernel-dependent/tree-independent state out of wrangler (#29)

* Register pytest markers

* Expansion wranglers: do not store trees

* Remove (unrealized) Wrangler.tree_dependent_info doc

* Add sumpy downstream CI

* Fix test failures for tree/wrangler refactor

* Create fmm.py

* More justification for TraversalAndWrangler design

* Placate flake8

* Back out ill-fated TraversalAndWrangler, introduce TreeIndependentDataForWrangler, introduce boxtree.{timing,constant_one}

* Fix pylint/flake8 for tree-indep data for wrangler

* Update the may-hold-tree comment in the wrangler docstring

* Fix incorrect merge of drive_fmm docstring

* Remove *zeros methods from wrangler interface

* Tweak sumpy downstream to use appropriate branch

* Fix downstream Github CI script

* Fix downstream CI script syntax

* Refactor so that FMMLibTreeIndependentDataForWrangler knows kernel but not Helmholtz k

* Adjust downstream pytential CI for wrangler-refactor branch

* Add template_ary arg to finalize_potentials

* Add {multipole,local}_expansions_view to wrangler interface