Use 'EXPECT_DOUBLE_EQ' for double type comparison
Documentation and variable name revision
Revise FemRepresentation::loadFile()
Keep 'face' for graphical part in ply file

Use lower case and underscore to separate names:
'1DElement' -> '1d_element'
'2DElement' -> '2d_element'
'3DElement' -> '3d_element'
Refactor part of registerDelegate into base class
Remove 'doLoadFile()', derived classes will return its delegate

Instead of passing the PlyReader to derived classes,
FemRepresentation::loadFile() will now ask its derived class to give
back the corresponding PlyReaderDelegate, and register this delegate and
do the load work.
Serialize Fem1D/2DRepresentation, add UnitTest