Calculate indices of ROI pixels corresponding to object locations.

def spatialBin(self, roi):
        """
        Calculate indices of ROI pixels corresponding to object locations.
        """
        if hasattr(self,'pixel_roi_index') and hasattr(self,'pixel'): 
            logger.warning('Catalog alread spatially binned')
            return

        # ADW: Not safe to set index = -1 (since it will access last entry); 
        # np.inf would be better...
        self.pixel = ang2pix(self.config['coords']['nside_pixel'],self.lon,self.lat)
        self.pixel_roi_index = roi.indexROI(self.lon,self.lat)

        logger.info("Found %i objects outside ROI"%(self.pixel_roi_index < 0).sum())