HVAC external capacity control

[mnblonsky]

Addresses #72

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[jmaguire1]

This would be useful for both limiting input power (AHRI 1380) as well as output capacity (for MPC type applications). Should we allow both? We also need to consider what happens to the backup element (disabled in AHRI 1380) and if you ever ignore the signal for comfort reasons (in the standard when you get x degrees off setpoint you override the schedule).

[mnblonsky]

Addressed parts of #72, but not all. "Capacity" control now allows user to directly set the HVAC capacity, and "Max Capacity Fraction" allows limiting the maximum capacity. But we don't have mass flow rate or supply temp yet.

[mnblonsky]

I'm skipping @jmaguire1's review since we're not merging into dev, and there's some bug fixes related to the new_controls branch that I'd like to fix ASAP. These changes should show up in the new_controls PR once that's done.