Add get_current_rss() method to determine the physical memory size of the process. This method is useful for debugging the memory usage of ctools.
Reduce memory requirements for ctobssim.

The memory needs for ctobssim have been reduced by first working on a local copy of the CTA observation (this de-allocates properly and memory cache associated to the response functions), and by saving and disposing the event lists immediately if called using the execute() method (this is the method called when the tool is run from the command line).
Add OpenPM critical zone for FITS file writing from within threads.
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.