Merge branch 'fix/boundingboxtree-update-in-problem' into 'master'

[bboxtree] Only call bounding box tree when there are point sources

When making the point source map we only want to make sure the bounding box
tree is initiated if there are any point sources. By a previous commit
this was mistakenly put in the beginning of the method so that the bounding
box tree is always built.

Maybe has to be tested with dumux-multidimension too.

See merge request !238