Change Trial_data model to belong to benchmark_id rather than task_id

[canjalal]

ACTUAL

If we delete a task (which belongs to both a benchmark and an assigned para) it will cause a referential integrity error in the database because trial data records belonging to that task will now point to a non-existent task.

EXPECTED

If we delete a task, it will not cause a referential integrity error because trial data is linked to benchmarks rather than tasks (which belong to a benchmark and assigned para).