Allow barostat temperature to be set via Context parameter, like pressure is

[jchodera]

If one wants to change the temperature and pressure of a simulation involving a System object with a MonteCarloBarostat, we can change the pressure on the fly via

parameter_name = barostat.Pressure() 
context.setParameter(parameter_name, pressure)

but the temperature used by the barostat must be changed via

barostat.setTemperature(thermodynamic_state.temperature)

which changes the serialized representation of the System object.

It would be useful if both temperature and pressure can be set via Contex global parameters.

See OpenMM feature tracker:
https://simtk.org/tracker/index.php?func=detail&aid=1979&group_id=161&atid=436

[jchodera]

This would still be useful!

[jchodera]

cc: #1440

[peastman]

We now support this.