ctools

I came across this bug while trying to jointly fit some VERITAS and HESS data recently. csbkgmodel allows passing an observation XML file with observations from multiple instruments. However, because there is a fitting portion of the script it will break when it tries to fit the generated background models to the list of observations. This appears to be because it doesn’t remove the observations from instruments not selected by the `instrument` parameter, which don’t have a corresponding model in the generated list of background models. This can be easily fixed by just removing those observations from `self.obs()` that are not associated with `instrument`:

        ...
        # Set instrument name
        self._instrument = self._get_instrument()

        # ===== new code
        # Remove observations from other instruments
        # This for-loop loops backwards through the observations
        for i in range(self.obs().size()-1, -1, -1):
            if self.obs()[i].instrument() != self._instrument:
                self.obs().remove(i)
        # ===== end new code

        # Query input parameters
        ...