Only try to establish connections at loading if such
are declared in the input file.
Avoid applying boundary cond. to self-connected planes + warn
+ link logger into StructureContainer.hpp
WIP
Some cleanup
Mark a problematic feature in StructureContainer.hpp

Making finite planes reflective by default if they are not connected
to others is dangerous: Particles that will dissociate close to the boundary
such that one of them reaches out of the plane will be reflected back,
causing overlaps! This has to be rethought!