Refactor batching of commands in Pipeliner

[pjbriggs]

Proposal to refactor the batching mechanism in the pipeliner module, which is currently done by inserting each subscript in a batch into its own block in a single "master" script.

The refactoring would involve generating separate script files for each subscript in the batch, and then creating a master runner script which would run each subscript in turn. This should be more robust and also provide a way to handle failure of one or more subscripts, without then stopping any subsequent subscripts from being executed.