Behavior for other SPARQL Update operations

[kjetilk]

As noted in #125, SPARQL Update contains several operations beyond DELETE and INSERT. It has been an open question what to do with them. Since they are not currently supported and thus falls outside of the supported subset, I suggest we simply disallow them on a SHOULD level.

I think we should review this soon with respect to COPY and MOVE in light of #19 , but I think this is the most straightforward way to address it right now.

I made this a separate PR to not tie up #320, but I hope we can get it in for completeness.