Add an option for CO2 setpoint to the honeybee_energy.load Setpoint class

[chriswmackey]

I sense that implementing this will be very similar to how humidistats are currently implemented for rooms that need tight humidity controls. We'll basically just write a ZoneContol:ContaminantController object in similar to how the ZoneControl:Humidistat is written in. More information about the use cases of this can be found here:

https://discourse.pollination.cloud/t/missing-parameters-in-rhino-gh-plugin/3565/18