mutli-valued property implementation needs GROUP BY

[ehedgehog]

When there are multi-valued properties in the dataset, the conversion of result sets to JSON relies on all rows with a specific ?item being together. This requires a SORT that orders on ?item or a GROUP BY likewise. The code has neither.

See also issue #29.

[ehedgehog]

Multi-valued search properties in the dataset force the dsapi to generate nested queries whose sole job is to find the selected items and limit them, with outer queries then adding the multiple rows
which will be merged by the dsapi code.