Remove temperature monitor hack

[alexdewar]

The temperatures are requested from the temperature monitor every 2s and they are used to a) update the plot and b) save into the data file (if recording is happening). The problem is that at some point Jon decided that he wanted data files to carry on recording regardless of whether the temperature monitor is active. In general this is a better design, because it makes things more resilient in the case of the temperature monitor breaking, but the way I implemented it was to add a hack that NaNs are broadcast in place of real temperatures if the device isn't working. A better way to do it would be to have all the sensors whose data can be recorded into data files respond to a different, specific message used for this purpose only.

[alexdewar]

I've just had a go at this and it turns out trying to pass all the different sensor values around using pubsub is a massive pain. The current solution is rather ugly, but I'm going to leave it in place for now until we can come up with a more elegant design.