Corrected link to the DAVE-ML exchange test [Jackson04] paper.

Added discrete enumeration selection to interpolate attribute of independentVarPts and independentVarRef elements at the suggestion of Geoff Brian, Australian Defence Science and Technology Organisation (DSTO).

Added a section and a variableDef example on extending the MathML-2 function set with atan2.

Removed all xns attributes from examples.

Emphasized that it is a good practice to provide variableDefs in sorted sequence.