Add support for parallel by DAGs

[mark-burnett]

This will not yet work for nested parallel by, because we need to split up the inputs as part of the split, the way we join outputs with the join code and not inside the input connector code.

[coveralls]

Coverage decreased (-0.54%) when pulling 9a568cc on mark-burnett:parallel-nested-dag into 9f2dcad on genome:dsl-update.

[davidlmorton]

+1 after the minor things I commented on.

I agree that we're going to need to have the task that "splits" on a parallelBy save some pointers to its inputs instead of trying to have the input_connector do it. Will the input_connector still need to make some pointers? I think it can just ask its task for inputs, once dags are Methods.

[coveralls]

Coverage decreased (-0.55%) when pulling f9bcacf on mark-burnett:parallel-nested-dag into 9f2dcad on genome:dsl-update.