Removed implicit prefix from Cucumber step definitions. Different identifier types are handled correctly. Reified relationships query no longer uses UNION.

As described in the title.

Note that although the reified relationships query no longer uses a UNION, I decided not to use COALESCE. This is because I needed to recognise and filter out blank nodes with non-item relationships (i.e. invalid). Using COALESCE would have required a second stage filtering of the initial result set.

Instead I made the join between 'from assertion' and 'to assertion' conditional such that 'to assertion' is always populated. In the case of reified relationships it points to the object relationship of the blank node. For non-reified relationships it's self referential.