Rework treatment of small and nominal values (#736)

- introduce system.eps_dp and system.eps_m_flow, along with system.dp_nominal and system.m_flow_nominal
- making system.dp_small and system.m_flow_small obsolete
- Interfaces.PartialTwoPortTransport: define nominal values in base class as they are needed by all derived classes since the introduction of homotopy
- Interfaces.PartialTwoPortTransport: define from_dp and dp_small in base class making common treatment more visible -- derived models can finalize unused parameters, e.g. valves to final from_dp=true
- Fittings, Pipes, Valves, Vessels: adapt to changes (so far by commenting out things that are now already defined in PartialTwoPortTransport)
- TestPipesAndValves.SeriesPipes1: rise time of filtered opening has been reduced from 1s to 0.1s, in accordance with other TestPipesAndValves models, in order to avoid an error as the valve approaches zero slowly 


git-svn-id: https://svn.modelica.org/projects/Modelica/trunk@5887 7ce873d0-865f-4ce7-a662-4bb36ea78beb