Added constructor for trace map for three-dimensional expansion

Remarks:
1) There is commented out part that deals with face orientation and reordering (it will be implemented once we finalize the face orientation notation)
2) NumDGBndryCoeffs is currently only implemented for Hexes


git-svn-id: https://gforge.sci.utah.edu/svn/nektar/trunk@3555 305cdda6-5ce1-45b3-a98d-dfc68c8b3305