FFT using pyvkfft and use loopy callables (#114)

* Use a separate class for M2L translation

* Fix docs and caching

* Fix p2p warning

* Use VkFFT for M2L generate data

* Fix profiling events

* simplify m2l data zeros

* Add pyvkfft to requirements

* Fix flake8 warning

* Fix typo

* VkFFT for M2L preprocess local

* vkfft for postprocess local

* Fix AggregateProfilingEvent

* Fix another typo

* 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

* remove whitespace on blank line

* Try 2r/order instead of r/order

* fix using updated pytential

* Fix tests

* use pytential branch with pyvkfft req

* Add explanation about caller being responsible for the FFT

* Fix for bessel

* Add pyvkfft to setup.py reqs

* use list comprehension

* Type annotations

* fix vim marker

* remove unused function

* m2l_data_inner -> m2l_data

* more descriptive name for child_knl

* knl -> expr_knl for clarity

* move loop unroll to optimized

* Add explanation about translation_classes_dependent_data_loopy_knl

* make coeffs output only and rewrite

* Re-arrange m2l so that event processing is easier

* flake8: single quotes -> double quotes

* Fix data not being input

* make args to cached_vkfft_app explicit

* cache vkfftapp in wrangler

* keep coeffs is_input and is_output for e2e

* out-of-place fft

* Use a separate queue for configuration

* allocate array for out-of-place

* fix typo

* Remove caching of opencl fft app

* Comment out pytentual fork

* fix vkfft queues

* use private API for now

* Add comment on pyvkfft PR

* remove inplace

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