Make get_current_rss() compile on Mac OS X.
Redeclare ctobssim::obs() as inline method.

As the GCTAObservation class now handles transparently the disposal and re-loading of information it is not needed anymore to re-load the event list upon call of the obs() method. We thus can put the obs() method inline as before.
Derive ctbkgcube Python module from ctool.
Reduce memory usage by disposing the event lists after usage.