Introduce reasonable default values to absolute and relative sensors (#948)

- replace zero specific enthalpy and composition with medium defaults
- introduce common base class PartialRelativeSensor for RelativePressure and RelativeTemperature
- update and rename TestOnePortSensors1 to TestTemperature1
- extend and rename TestOnePortSensors2 to TestTemperature2 -- this example is likely to fail with h_outflow=0
- delete TestTemperatureSensor as duplicate of Modelica.Fluid.Examples.Explanatory.MeasuringTemperature


git-svn-id: https://svn.modelica.org/projects/Modelica/trunk@5863 7ce873d0-865f-4ce7-a662-4bb36ea78beb