fix query evaluation bug affecting nested within expressions

The evaluation bug caused query statements like
(8.0 <> [cname=A] and cname!=A) or 8.0 <> ([cname=B] and cname!=B)
to be misinterpreted. Fixes BZDNG-95.

git-svn-id: https://dng.biozentrum.unibas.ch/svn/openstructure/trunk@1875 5a81b35b-ba03-0410-adc8-b2c5c5119f08