Throw exceptions if response information is missing.

So far the GCTAResponse class simply returned a zero response in case that no response information was loaded. This is not very informative for a user, who may debug for a long time before recognizing that he forgot to load response information.

Now, exceptions are thrown if response information is missing, telling the user that he has to load the response properly. This should be much saver.
Implement flexible handling of XML observation definition files for CTA, including specification of the calibration database and response name.

The XML observation definition file for CTA does no longer require the IRF file parameters. A simple event list or counts map is sufficient. Calibration parameters are optional. A first option allows specifying the calibration database and response name using the "Calibration" parameter. Another option consists of loading specific files (this is the old format).
Remove optional instrument response parameters from CTA observation definition files.
Use CTA observation definition file containing response information
Use dummy response instead of 2D response (which has not yet energy dispersion).
Adjust input file to get correct response information.
Add index file and adjust calibration database destination path.