Add consturctor to convert pointer types
add GCTAExposure and GCTAMeanPsf classes
add test script
Add cast constructor to Python interface and remove it from C++ interface.
Enforce coding conventions and refactor code.
Add read(), write(), load() and save() methods to allow storing and loading of nodes in a FITS file.
Correct for some compilation problems and add writing of deltas to PSF cube class.
Set clobber default to false.
Remove obs argument in exposure cube constructor.

Conceptually it is better to separate exposure cube definition from exposure cube filling. This is now done.
Add exposure cube unit tests.
Separate PSF cube filling from PSF cube definition in constructor.

The observation argument has been dropped in the PSF cube constructor.
Implement PSF cube unit test and change delta angle binning from a quadratic to a linear scale.
Merge branch '1233-combine_irf' into integration
Remove intermediate testing script (unit tests are now implemented in test_CTA).
Update change log and News after initial GCTAExposure and GCTAMeanPsf implementation.