Allow setting `dl1_files_per_job` in the `r0_to_dl1` and `dl1ab` stages

To be implemented into the `base_config_lstmcpipe.yaml` ? Or at least it should be configurable somehow.

https://github.com/cta-observatory/lstmcpipe/blob/master/lstmcpipe/stages/mc_process_dl1.py#L179