some small improvements in comments and error handling

git-svn-id: http://svn.code.sf.net/p/sumo/code/trunk/sumo@15491 29f149b2-0c63-11e1-8f5c-a347f6a9acd0
adding a pie chart plotting tool

git-svn-id: http://svn.code.sf.net/p/sumo/code/trunk/sumo@15492 29f149b2-0c63-11e1-8f5c-a347f6a9acd0
adding some basic tests for plotting tools (pie chart only by now)

git-svn-id: http://svn.code.sf.net/p/sumo/code/trunk/sumo@15493 29f149b2-0c63-11e1-8f5c-a347f6a9acd0
small improvements in docs

git-svn-id: http://svn.code.sf.net/p/sumo/code/trunk/sumo@15494 29f149b2-0c63-11e1-8f5c-a347f6a9acd0
disable net validation (probably only useful for broken xerces versions), refs #1135

git-svn-id: http://svn.code.sf.net/p/sumo/code/trunk/sumo@15495 29f149b2-0c63-11e1-8f5c-a347f6a9acd0
adapting meta tests to new command line option, refs #21, #1135

git-svn-id: http://svn.code.sf.net/p/sumo/code/trunk/sumo@15496 29f149b2-0c63-11e1-8f5c-a347f6a9acd0