ATR-21589: Automate the merging strategy

Heather Russellrequested to merge
hrussell/athena:TM-ATR-21589_merging2 into master

Automatically parse the merging strategy from the alignment dictionary.

Set the default merging to 'auto'.

Remove merging='parallel'/'serial' from all chains in the menu. Changed newJO menu to include 'parallel'.

Auto-merging strategy is setup to allow for chains needing a hybrid strategy, though the serial merging explicitly disallows this case: opened ATR-22206 to follow up.

Edited by Heather Russell

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