change optional match syntax in query

[superfell]

Query supports an optional match operator, however the syntax is subtle (a trailing ? on the predicate) and the part of the query that is optional is determined by that query lines subject & object variables. This gets more confusing if there are multiple optional matches. This should be moved to Sparqls optional match format where its much clearer both that there is an optional match, and what the optional match is made up of.

[darrengarvey]

The current syntax is a standard SPARQL 1.1 property path. OPTIONAL {} is still useful for optional blocks of query lines.