PID algorithm for temperature control

[davidsean]

may be related to #1 but one may consider using a PID control algorithm to stabilize environmental conditions.

[davidsean]

After offline discussion with @timsp and @shadjiantoniou, we all think his will not be possible to control gas levels with the current hardware. Unless we upgrade to a modulating control valve (as opposed to the on/off ones), this should be dropped for gas control.

However, temperature is still possible and needs to be tested.