Fix/GetBoundaryToElemtMap3 dh1 d consistency

This MR changes the ordering from DisContField3DHomogeneous1D::GetBoundaryToElmtMap to make it consistent with m_bndCondExpansions (i.e. sorting by boundary, instead of sorting by plane). This allows the boundary expansion from 3DH1D to be treated in the same way as the non-homogeneous case, reducing the need for expansion-specific code.

All parts of the code assuming the old ordering were updated to take these changes into account.