Improve and harmonize automatic detection of dp_small and dp_nominal (if system.use_small==false) (#736)

System: 
  - change default value of system.use_small to true for backwards compatibility with MSL 3.2
Interfaces: 
  - revert extension from r5887 as no useful default values are known in the base class
Pipes, (old) Fittings, Valves:
  - declare dp_small protected as it is determined automatically from m_flow_small 
    (cf. new Fittings basing on Dissipation)
Pipes, Fittings.mo(QuadraticTurbulent): 
  - introduce automatic determination of dp_nominal=f(m_flow_nominal) 

Examples.IncompressibleFluidNetwork: change to new system.use_small=false  
  - avoid wrong simulation results if medium Essotherm650 is used
  - model using TurbulentPipeFlow works for alternative medium StandardWaterOnePhase
Examples.HeatExchanger: 
  - change to new system.use_small=false as simulation failed after bug fix in Pipes
    (Pipes.BaseClasses.FlowModels.PartialGenericPipeFlow: dp_small/(n-1) instead of dp_small/n)

ModelicaTest.Fluid:
  - remove heuristic dp_small/dp_nominal settings and use automatic detection instead
    (system.use_small=false)


git-svn-id: https://svn.modelica.org/projects/Modelica/trunk@5918 7ce873d0-865f-4ce7-a662-4bb36ea78beb