If there is a point in the list with x-coordinate x, then this function returns
the index of that point in the list (where the index of the first point is zero).
If an edit function has been specified (by startEdit()), this function copies the
data form the TableFunctionInput into that function, and returns a
pointer to that function.
An object belonging to a concrete subclass of FunctionParserExtesion is a
mathematical function that can be registered with a Parser and then used in
strings that are parsed by that parser.