Merge branch 'feature/improve-pointsources' into 'master'

Feature/improve pointsources

* Make convenience assignment operators work for the classes derived from PointSource
* Make the PointSourceHelper (computing in which scv a point source falls) a property to be able to exchange implementations (the current default uses the bounding box tree. This is however a waste of resources if one already knows more about where the point source is.)
* Rename PointSourceHelper to BoundingBoxTreePointSourceHelper

See merge request !134