Add "barely opened" position to Valve class

Add "barely opened" position to Valve class

When measuring solubility of gasses in liquids, opening of the valve and
the resulting movement of gas may upset the level of the liquid to the
point of blowing droplets somewhere, where they cause problems (mainly
weighing of the sample after measurement).
Opening the Valve barely to make equilibration of the pressures take
more time should prevent the violent gas flow.