OM Refactor: Enable multiple cluster functionality

[elenya-grant]

OM Refactor: Enable multiple cluster functionality

This PR enables the ability to run multiple clusters. The main additions of this are connecting the controller to multiple clusters and also aggregating performance across clusters with the SystemPerformance component.

This also includes some clean-ups to some of the methods in bert.py and introduces a new input to BERT - which is as_problem. If as_problem is True - then the plant group is added to an OpenMDAO problem. If as_problem is False, the plant group is just created as a group and not added to an OpenMDAO problem. This addition is helpful for wrapping the BERT model in other openmdao models.

Also updated the dynamics so that I_out is multiplied by on_off_status and so n_stacks and n_cells is promoted to the top-level.

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