Add explicit forward declaration
Move call to 'loadFile()' into FemRepresentation::initialize()
Derived class has to customize 'beginVertices()' and 'processVertex()'
Fem3DRepresentationReader UnitTest fix
Use 'tetrahedron' instead of 'polyhedron' for Fem3D

'tetradehron' is conceptually more accurate than 'polyhedron' in the
sense of Fem3D.
Implement PlyReaderDelegate for Fem2DRepresentation
Implement PlyReaderDelegate for Fem1D
Use 'tetradeheron' instead of 'polyhedron' for Fem3D data