Issue39 ideal heater cooler

The pull request by @nytschgeusen had two committs: renaming `u` to `Tset` and moving `TemperatureControlledHeatFlow`.
Renaming had already been done by @mwetter (only forgot one u in the documentation) in the issue39 branch.
This pull request implements the same changes as the pull request by @nytschgeusen but starting from the latest committ in the issue39 branch.
If you prefer, you can reject and just move TemperatureControlledHeatFlow yourself.