Evaluates the given XPath expression.
Two arguments are required: the expression (as a string or compiled
expression object), and a context. The context can be given as a
Domlette node via the 'contextNode' named argument, or can be given as
an Ft.Xml.XPath.Context.Context object via the 'context' named
argument.
If namespace bindings or variable bindings are needed, use a
Context object. If extension functions are needed, either use a
Context object, or set the EXTMODULES environment variable to be a
':'-separated list of names of Python modules that implement
extension functions.
The return value will be one of the following:
node-set: list of Domlette node objects (xml.dom.Node based);
string: Unicode string type;
number: float type;
boolean: Ft.Lib.boolean C extension object;
or a non-XPath object (i.e. as returned by an extension function).