Added option to hide op declarations and fixed write_schema/2.

Hi,
I've just been trying out your rdfs2pl pack and found a little bug in write_schema/3 which was preventing it from working with a namespace/prefix that was already defined. I also added an option to prevent operator declarations and a couple of minor fixes to get rid of some warnings. In particular, I assumed that the call to inf_clause/2 should have been a call to inf_clause/3 with no options.
Thanks for the code!
Samer.