message: Add some documentation to PointMatrix.
diff: @@ -761,6 +761,19 @@ protected: // implementation
  }
  };
  + /** @brief The PointMatrix stores the coordinates of the slave face using the
  + master face coordinate as reference.
  +
  + In 2D, the point matrix has the orientation of the parent
  + edge, so its columns need to be flipped when applying it, see
  + ApplyLocalSlaveTransformation.
  + In 3D, the orientation part of Elem2Inf is encoded in the point
  + matrix. The following transformation gives the relation betwen the
  + reference face coordinates (xi, eta) in [0,1]^2, and the fine face
  + coordinates (x, y):
  + x = a0*(1-xi)*(1-eta) + a1*xi*(1-eta) + a2*xi*eta + a3*(1-xi)*eta
  + y = b0*(1-xi)*(1-eta) + b1*xi*(1-eta) + b2*xi*eta + b3*(1-xi)*eta
  + */
  struct PointMatrix
  {
  int np;
  