feat: PneumaticPressureLoader selectable blowout source

[pbeaucage]

PneumaticPressureLoader should offer the option to use either a solenoid valve or a pressure controller as a source for blowout commands, as an option. The pressure controllers can typically deliver (much) higher blowout pressures than the dispensing system, which is usually limited at 15 psig (1 atm) for safety of glass bottles used to dispense.

[pbeaucage]

This was sigh already implemented.

If a load step name is ctrlblow, then the loader will command the pressure controller to self.config['blowout_pressure'] and back to zero at step end.