Register a new function.
By registering a new function, any XPath expression that utilizes this FunctionContext
may refer-to and use the new function.
Functions may exist either in an namespace or not. Namespace prefix-to-URI resolution is the responsibility of a {@link NamespaceContext}. Within this FunctionContext
functions are only referenced using the URI, not the prefix.
The namespace URI of a function may be null
to indicate that it exists without a namespace.
@param namespaceURI The namespace URI of the function tobe registered with this context.
@param localName The non-prefixed local portion of thefunction to be registered with this context.
@param function A {@link Function} implementation objectto be used when evaluating the function.