code: def sparse_counts_map(self):
          
          if self.hpx._ipix is None:
              flatarray = self.data.flattern()
          else:
              flatarray = self.expanded_counts_map()
          nz = flatarray.nonzero()[0]
          data_out = flatarray[nz]
          return (nz, data_out)
docs: return a counts map with sparse index scheme