adding virtual destructor refs #12

git-svn-id: http://svn.code.sf.net/p/sumo/code/trunk/sumo@17033 29f149b2-0c63-11e1-8f5c-a347f6a9acd0
fixing broken gcc build (clang did not complain). Not sure whether the method is properly inherited now. refs #3

git-svn-id: http://svn.code.sf.net/p/sumo/code/trunk/sumo@17034 29f149b2-0c63-11e1-8f5c-a347f6a9acd0