ENH: Batching

Supports batching based on:

1. user-provided label (i.e., batch keyword while adding job to pipeline)
2. fsl_sub parameters; only jobs with identical parameters (except for jobtime, name and optionally jobram) are batched.

By default only connected jobs are batched (see only_connected keyword when calling batch).

TODO:

• create submit_params for combined job
• create nice name for combined job