Add list of abstract functions to XML representation of model

[zirkel]

The model builder creates an AbstractFunction for each abstract function in the code. These are used by derivatives and evaluated function expressions. An abstract function has a name, return type, array of argument types, and continuity value. The XML model needs a list of all of the abstract functions at the beginning, similar to the list of types.

[ywei]

Abstract function list added in the XML schema.