message: Credit to MFEM + link to reference.
diff: ///
  /// Action of 3D Mass matrix via partial assembly
  ///
  -/// TODO add description
  +/// Based on MFEM's/CEED algorithms.
  +/// Reference implementation
  +/// https://github.com/mfem/mfem/blob/master/fem/bilininteg_mass_pa.cpp#L925
  ///
  ///
  