Rename to FemRepresentationCoordinate::naturalCoordinate

The generalized term for coordinates in fem is "natural coordinate", not
"barycentric coordinate".
Add isValidCoordinate function to fem and femElement

- FemElement::isValidCoordinate checks whether the naturalCoordinate is
properly formed
- FemRepresentation::isValidCoordinate combines FemElement's check with
an additional check for whether the elementId is valid
- Checks whether coordinate is valid in
Fem3DRepresentationLocalization::setLocalPosition
Refactoring in fem tetrahedron and fem localization test

- Use getSubVector, reduce calculations
Initialize variables in FemRepresentationCoordinate constructor
Update documentation in fem localization and fem coordinate