* Add SpatialConstant

* Add docs

* Improve docs around SpatialConstant, ExpressionKernel

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

* 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>

* 4r/order for biharmonic

* Add a test for magnitudes

* Use a separate class for M2L translation

* Fix docs and caching

* M2L Translation Factory

* vim markers

* Fix tests

* Fix toys

* Fix test_m2l_toeplitz

* Fix more tests

* Use a better rscale to get the test passing

* Use pytential dev branch

* Try 2r/order instead of r/order

* Compute translation_classes_data if not given

* Remove SumpyTranslationClassesData

* update tests

* use readable syntax

* Use a better rscale from Greengard 1988

* Use pytential dev commit

* remove level_to_rscale function

* Use pytential branch only for rscale sumpy branch

* Fix HelmholtzDerivativeTaker

* Use 2, 3, 4 for 2D

* Divide by r**2 at the end

* Write the recursion as one expression

* post process kernel

* Fix postprocesslocal

* use preprocess/postprocess everywhere

* Fix event management for m2l

* Use a loopy kernel

* return a loopy kernel

* fix formatting

* reduce diff

* Fix typo

* Fix typo

* use_preprocessing_for_m2l -> use_fft_for_m2l

* supports_optimized_m2l -> supports_translation_classes

* restore non fft optimized code path

* events -> timing_events

* More descriptive exception message

* use lp.Assignment instead of strings

* Fix typo

* Add a comment about symbolic sum

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

* Fix error message

* split too long line

* Make use_preprocessing_for_m2l an option

* Add an comment about the options

* fix syntax errors

* add missing defines

* Fix hashing

* Fix returning nexprs

* fix use_preprocessing_for_m2l

* Fix ncoeff_src and ncoeff_tgt

* fix use_preprocessing_for_m2l for bessel based

* Fix logic

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