Move FemRepresentationCoordinate into new h/cpp

- There should be one header file per class.
- In preparation for next commit (renaming FemRepresentationLocalization
to Fem3DRepresentationLocalization).
Rename to Fem3DRepresentationLocalization

Fem3DRepresentationLocalization::doCalculatePosition and
::isValidRepresentation depend on casting a Representation to a concrete
implementation.  The previous FemRepresentationLocalization actually
only worked for Fem3DRepresentation, and the new name reflects that
fact.
Fix cpplint warnings