Various tweaks to clean up code

- Constructor signatures for KeyedVector have been swapped to ensure
consistency in parameters between the overloaded constructors.
- KeyedVector now uses the Linq library
- Point renamed to CartesianPoint
- signature of abstract methods in ElementStiffnessMatrixBuilder have
been amended to refer to local coordinate system.
- Ignore assertions are now Inconclusive where the test was built but
the data for testing against isn't available or verified.