Revise using eigen axis unit vectors.
Add documentation for EmptyData class.
Removing unused included header files of EmptyData in TriangleMesh.h
and MeshShape.cpp
Clean up code and remove 'using' in header files.
Add a dedicated constructor for MeshElement that has empty data member.
Revise the code with new dedicated constructor of MeshElement that
does not need data member.
Remove EmptyData type in MeshElement class.
Fix bug, remove normalized() for unit vector in makeRotationQuaternion and
also add UnitY().
Remove normalize().