add n_repeats option to task-based setup

[atravitz]

when creating a task graph from an alchemical network (here), we should add an option to create multiple repeats with an n_repeats:int argument.

[atravitz]

I'm actually wondering if this is something we should add for v1.13, or if we should encourage users to use the n_repeats arguement in settings, since they would be functionally equivalent.

What might be more useful is an --amend flag to setup-task-campaign that allows a user to add tasks to an existing warehouse/taskdb campaign.

[IAlibay]

I would suggest we move this to v1.14 and for now just have folks just use the protocol default settings.

Medium term there's a discussion to be had since we probably want in-protocol repeats to be functionally different from "complete" repeats.