Add support for XPath's function calls in XML query

[yaozhihang]

Currently, the XPath's function call e.g. count is not supported in XML query expression and the query like "Find all buildings having at least four wall surfaces" cannot be expressed. We could use the generic SQL query as a fallback, but it would be nice to have the support of XPath's function calls.

[clausnagel]

Will not be resolved for version 5.x of the Importer/Exporter. A SQL query can be used as fallback.