fix import of compounds with only one atom

components.cif does not use a loop_ section if the residue only
has one atom. Now we also handle these single atom residues
properly

git-svn-id: https://dng.biozentrum.unibas.ch/svn/openstructure/trunk@2214 5a81b35b-ba03-0410-adc8-b2c5c5119f08