cleanup for PDBReader

Particularly, split out the common part of ANISOU, HETATM and
ATOM parsing into a separate method. Switch the line processing
foo to use StringRef, which makes it look much simpler, plus
gives some speed improvements as we avoid copying around
std::strings.

This should have been done a long, long time ago.

git-svn-id: https://dng.biozentrum.unibas.ch/svn/openstructure/trunk@2039 5a81b35b-ba03-0410-adc8-b2c5c5119f08