Make taylor series with FFT the default for Helmholtz and Biharmonic (#120)

* Make taylor series the default for Helmholtz

* Try 4r for Helmholtz

* Update fmm.py

* Use 4r/order for biharmonic too

* Add missing import

* Update pytential commit

* Revert "Update pytential commit"

This reverts commit 9f3b25de4d031d187da8e3ddaaaad79a6c730a61.

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