Support for CONSTRUCT and DESCRIBE in pipelines

[edwardsph]

Both these types of queries can return XML formats so would fit well in an XProc pipeline. At present the calli:sparql function only allows SELECT queries. I would suggest extending the capabilities of calli:sparql to support these query types rather than add new functions since they are all sparql. If it were necessary to create new functions such as calli:construct and calli:describe, it would suggest the existing function ought to be renamed calli:select.

If this work required adding support for these queries in NamedQueries, I think that would be a good thing too.