* Support M2L with FFT on ToyContext

* flake8 fixes

* Fix src_rscale, tgt_rscale ordering

* More tests with FFT

* Use sumpy.toys in test_translations

* flake8 fixes

* 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

* Fix HelmholtzDerivativeTaker

* Use 2, 3, 4 for 2D

* Divide by r**2 at the end

* Write the recursion as one expression

They are aggregated using a strength expression