⛔ [DO NOT MERGE] ⛔ Fix partition grouping bug ⛔

[mcovarr]

⛔ DO NOT MERGE this into the EchoCallset branch ⛔ just yet; it's probably better to address this bug in the current EchoCallset run by adding the two missing partitions to the last group and rerunning the last group only rather than rerunning all the groups with a larger group size.

Add in n_rounds - 1 to the group size expression to include all of the partitions in the set of groups, otherwise we omit the final n_parts % n_rounds partitions.

Concretely for AoU Echo with 145192 total partitions and 5 rounds:

>>> 145192 // 5
29038
>>> 29038 * 5
145190
>>> (145192 + 5 - 1) // 5
29039

[gbggrant]

FYI - I merged the same code (in a different PR) into EchoCallset branch.