set zenith in GCTAPoinitng
Add fixed_iter() method and implement fixed iteration handling in Romberg method.
Add left_difference() and right_difference() methods.
Add romberg() method variant that takes a vector of (unsorted) boundaries. The method will sort these boundaries and returns the sum of the integral over all boundaries. This allows easy splitting of an integration range into two pieces by adding an internal node.
Adapt model and Npred gradient computation to cope correctly with parameter boundaries. Also streamlined the interface to avoid passing parameter indices. We now pass parameter references.
Adapt CTA response integration helper classes to a fixed number of iterations.

Due to numerical stability it is adviced to use a fixed number of integration iterations instead of fixing the integration precision. The latter leads to substantial numerical noise that make computing of numerical derivates very difficult.
Modify response computation so that a fixed integration iteration depth is used instead of a given precision.
Remove integration precision eps().
Now runs full benchmark test over all model types and analysis variants.
Remove GCTAResponseIrf::eps() method.

This method is now obsolete as we do not use integration precisions anymore.
Fit all model types and analysis methods.
Change GIntergal::romb() to GIntergal::romberg().
Adjust reference values after implementing smooth Psf edges.
Update method.
Update following recent changes.
Add shell model visualization.
Ignore any test data under inst/cta/test.
Correct typo in Doxygen documentation.
Remove Psf cube that was accidentally committed.
Correct implementation of fixed iteration scheme.

The fixed number of iterations has not been correctly taken into account in the convergence testing. If alternatively the integration precision was reached, the code stopped anyways.