shorten methods of GenericPropertyContainer somewhat

The old signatures are still available in Python, but produce
a deprecation warning. After some time, we will remove the
long signatures completely.

git-svn-id: https://dng.biozentrum.unibas.ch/svn/openstructure/trunk@2113 5a81b35b-ba03-0410-adc8-b2c5c5119f08