improved behaviour of rule-based builder

as soon as an atom is unknown, we switch back to a distance-based
connect mechanism for the full residue. This avoids problems with
residues coming from CHARMM/GROMACS and other force fields that do
not follow the naming convention of the PDB. To really become useful,
some additional changes to the compound library are required that
will be implemented in a next commit

git-svn-id: https://dng.biozentrum.unibas.ch/svn/openstructure/trunk@2714 5a81b35b-ba03-0410-adc8-b2c5c5119f08