[GPU] Create a better app that uses an STL for a geometry

Currently the only app with an STL for a geometry is the SphereInChannelApp.

This app also has a parameter that allows to change the size of the sphere. This does not work when using the STL. I think that this is contradictory and confusing for users.

I suggest that we us a geometric primitive for the sphere in the sphere app. If we do this now, we don't have an app with a stl geometry anymore. Therefore I suggest that we add a new app with an stl.

Any ideas? @m.schoenherr @soe.peters