Make heatCoolAmountProperty read-only on all screens

[arouinfar]

For #30

The heatCoolAmountProperty is read-only on the Ideal Screen, as the model needs to automatically heat/cool, depending on the Hold Contant mode. However, it is API-settable on the Explore and Energy screens. We made a design decision for the HeaterCoolerNode to snap-to-zero when not user-controlled, and we don't allow the snap-to-zero behavior to be turned off with a query parameter, as we do in EFAC and SoM. Allowing clients to control heatCoolAmountProperty is inconsistent with these decisions.

@pixelzoom please make heatCoolAmountProperty read-only on all screens.

[pixelzoom]

heatCoolAmountProperty is now read-only for all screens. @arouinfar please review, close if OK.

[arouinfar]

Thanks @pixelzoom. Looks good on main, closing.