ctools

While it seems logical that ctbkgmodel aborts if no background model is found, there could be a workflow where there is no background model, and consequently, the output files should be an empty cube. This is particularly important since tools may later request a background cube, although it may not be needed.

The logic should be changed so that ctbkgmodel does not abort in the absence of a background model, producing output that can be digested by subsequent tools, such as ctlike.