ctools

I guess we already have this in ctbin. Here we check for the sanity of the RoI:

// Check for RoI sanity
if (!roi.is_valid()) {
   std::string msg = "No RoI information found in input observation " 
                      "\""+obs->name()+"\". Run ctselect to specify " 
                      "an RoI for this observation before running " 
                      "ctbin.";
    throw GException::invalid_value(G_FILL_CUBE, msg);
}

I guess, we only need to take these lines of code and implement them in GCTACubeExposure::fill(), GCTACubePsf::fill(), GCTACubeBackground::fill(). I would do it right now if I don’t had a pending pull request on those files (see #1439)