Add a valve model

This fixes #57 by adding a base class of a valve which then in turn is used by a new valve model and a modified simple turbine model.

@boerrebj I'm not expecting you to go through this in detail but if you could just take a quick look at OpenHPL.Examples.SimpleValve and how the new OpenHPL.Waterway.Valve is now using OpenHPL.ElectroMech.BaseClasses.BaseValve and if you think this is OK. Then you can approve it :-)