docs: def get_qpimage(self, idx):
          """Return background-corrected QPImage of data at index `idx`"""
          # raw data
          qpi = self.get_qpimage_raw(idx)
          if "identifier" not in qpi:
              msg = "`get_qpimage_raw` does not set 'identifier' " \
                    + "in class '{}'!".format(self.__class__)
              raise KeyError(msg)
          # bg data
          if self._bgdata:
              if len(self._bgdata) == 1:
                  # One background for all
                  bgidx = 0
              else:
                  bgidx = idx
  
              if isinstance(self._bgdata, SeriesData):
                  # `get_qpimage` does take `idx`
                  bg = self._bgdata.get_qpimage_raw(bgidx)
              else:
                  # `self._bgdata` is a QPImage
                  bg = self._bgdata[bgidx]
              qpi.set_bg_data(bg_data=bg)
          return qpi