Refactor E2P and P2E (#153)

* Use pytential branch

* Refactor E2P

* try new loopy branch

* fix formatting

* disable domains check

* register only if not found

* Move kernel_scaling to the outer kernel

* Refactor P2E

* Use loopy main

* re-enable implemented domains check

* Rename some loopy kernel handling functions

---------

Co-authored-by: Andreas Kloeckner <inform@tiker.net>