Revert --batch change for pipeline_job.sh

[holtgrewe]

Is your feature request related to a problem? Please describe.
One cannot run multiple batches at the same time (the input of a batch may overlap, of course).

Describe the solution you'd like
Remove change to pipeline_job.sh again. Rather, allow to specify an optional SNAPPY_BATCH.

Describe alternatives you've considered
N/A

Additional context
N/A

[holtgrewe]

We can use the following snippet:

# Allow selection of batch
if [[ ! -z "${SNAPPY_BATCH-}" ]]; then
    SNAKEMAKE_BATCH_ARG="--batch ${SNAKEMAKE_BATCH_RULE-default}=${SNAPPY_BATCH}"
else
    SNAKEMAKE_BATCH_ARG=
fi