Add apply_thetacut() method to GCTAAeffArf class and call this method after loading IRF information so that the thetacut is applied correctly to effective areas provided in the ARF format.

The thetacut application has been removed after response factorization, leading to fit results that differ substantially from the results found for 1DC analysis using gammalib-00-05-00. Now, using the apply_thetacut() method essentially the same results are found.