Generalize process id producer and stitching configuration and include stitching for w_lnu samples

[pkausw]

This PR generalizes the modules relevant for stitching samples in the analysis. The main differences are two-fold:

1. The process id producer is now more generalized to account for different methods of stitching. While this is probably overkill while we only stitch DY and W+jets samples, this might be useful in the future
2. the config entry has a more generalized structure:

cfg.x.stitching = {
            "DATASET_TAG": {  # e.g. for DY datasets, this would be 'is_dy'
                "base_producer": PRODUCER_TO_USE,  # e.g. for DY, this would be `process_ids_dy`
                "info": {
                    "m50toinf": {
                        "inclusive_dataset": INCLUSIVE_DATASET,   # e.g. for DY, this would be `cfg.datasets.n.dy_m50toinf_amcatnlo`
                        "leaf_processes": [
                            # the following processes cover the full njet and pt phasespace
                            # for DY:
                            # procs.n.dy_m50toinf_0j,
                            # *(
                            #    procs.get(f"dy_m50toinf_{nj}j_pt{pt}")
                            #    for nj in [1, 2]
                            #    for pt in ["0to40", "40to100", "100to200", "200to400", "400to600", "600toinf"]
                            # ),
                            # procs.n.dy_m50toinf_ge3j,
                        ],
                    },
                }
            },
}

This way, we can configure all stitchings in one place. Tested with w_lnu and dy datasets