Cooling flag support

[glynhudson]

A number of systems are being used for cooling.

It would be great to have a field where the user could select a boolean cooling flag in the same was as we currently have flags for CH and DHW.

The proportion of time that the system spends in cooling could then be highlighted on the graph and the cooling heat counted as positive heat when the cooling flag is present since it's intentional cooling, as opposed to unintentional defrost cooling.

[glynhudson]

This system has cooling usage with negative heat production: https://heatpumpmonitor.org/system/edit?id=275

This system has cooling usage with the Input processing setup to invert the heat power when cooling is present: https://heatpumpmonitor.org/system/view?id=326