Support TargetPointMultiplier (#75)

* Use TransformationRemover

* Use itgt to index targets

* src->sources, tgt->targets

* Use sumpy wip branch

* Fix typo

* Fix center_itgt->itgt

* Fix LayerPotentialOnTargetAndCenterSubset to use targets

* pass itgt_name

* Remove whitespace on blank line

* Fix get_optimized_kernel

* sdr, tdr -> sxr, txr

* Point req.txt for sumpy back to main

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