Add TargetPointMultiplier (#46)
* Add TargetPointMultiplier
* use *TransformationRemover instead of *DerivativeRemover
* Fix TargetPointMultiplier and add a test
* Fix qbx
* Fix rename of src->sources
* use pytential branch
* make itgt_name customizable
* Fix itgt_inner
* sdr -> sxr
* Explain itgt_name override in QBX
Co-authored-by: Andreas Klöckner <inform@tiker.net>