ResampleStep.single ignored

[braingram]

Running resample with --single=True does not run many_to_many:

strun romancal.step.ResampleStep --single=True <some_asn.json>

outputs the appropriate step spec (with single=True)

  single: True

but then calls many_to_one and does not produce an output for each group.

I suggest that we remove this from the spec (rather than changing it to run many_to_many) for the following reasons:

• many_to_many results are incomplete, they do not contain resampled variance, exposure time, etc
• a more complete output can be created by generating associations for each group and running them with the default single=False
• this would allow removal of many_to_many (at the moment this is used by outlier detection but an alternative can be found that produces the same result without requiring use of many_to_many, I'm happy to give more details but this is currently a work-in-progress and isn't the main focus of this issue)