summarypipe --rundir for pbilby: dealing with multiple result files and inis

Hi @charlie.hoy !459 (merged) looked super useful to me at first glance, particularly since you report @virginia.demilio had tested it on parallel_bilby runs as well. However, when I do that on one of my runs, it runs into two "multiple file" issues:

1. Multiple result files found and then it decides to use (going alphabetically, I guess) the first single-seed 0_result.json instead of the merged_result.json which would be the natural choice. I think it should be hardcoded to, if encountering multiple files, try for a file ending in that str first, and else fail and ask the user to specify one, similar to the config situation.

2. ValueError: Multiple configuration files found as you also reported in your bilby_pipe example on the MR; so I could pass the right one manually as you suggest, however I think this is actually the generic situation with parallel_bilby and should get a hardcoded workaround: the way we set up parallel_bilby runs, we usually have a hand-edited config file, then call parallel_bilby_generation on it, and that produces an additional tag_config_complete.ini file. So every rundir will have those two files. If that situation is encountered, I'd suggest to default to the one that does not end on complete.ini, because that one omits the prior file information and is hence less useful for reproducibility of the run from the pesummary page. (parallel_bilby devs may comment on this, though - cc @gregory.ashton @rory-smith)

I'm not really ever using non-parallel bilby, so if any of what I suggest would lead to a problem for those runs and hence can't be implemented as global defaults, an extra switch to tell it it's looking at a parallel rundir would be a more convenient solution than having to hand-specify --pattern and/or --config to work around those two issues.