updated logging system

specifically, remove LOGN_* macros and introduce new logging levels. We now
have ERROR, WARNING, INFO, VERBOSE, DEBUG and TRACE. DEBUG and TRACE are
turned off under NDEBUG and expand to an empty macro.

new default verbosity level displays ERROR/WARNING/INFO

git-svn-id: https://dng.biozentrum.unibas.ch/svn/openstructure/trunk@2709 5a81b35b-ba03-0410-adc8-b2c5c5119f08