Added min, start and nominal attributes.
Revised implementation of heat exchanger base classes.

This revision reverses the syntax change of
{Two,Four}PortHeatMassExchanger to use the construct 'constrainedby'.
This avoids a warning in Dymola.
The revision also simplifies the implementation of the wet coil.
Merge pull request #178 from lbl-srg/openmodelica

Merge changes for OpenModelica
Cleaned up code formatting.
Removed unrequired model MixingVolumeDryAir
Revised example models for MixingVolumes.
Added start value for hOut to avoid a warning in OpenModelica.
Removed base class Buildings.Fluid.MixingVolumes.BaseClasses.PartialMixingVolumeWaterPort.
Added missing each keyword in parameter declarations