Remove dp(start=dpValve_nominal/2) in PartialThreeWayValve

[mwetter]

The model Buildings/Fluid/Actuators/BaseClasses/PartialThreeWayValve.mo is the only one where the start attribute of dp is modified. It is set to dp(start=dpValve_nominal/2). The two-way valves, and the fixed resistances, do not set this start value, and hence it remains at 0 as set here:

modelica-buildings/Buildings/Fluid/Interfaces/PartialTwoPortInterface.mo

Line 24 in 21859cb

start=_dp_start,

This issue is to remove the assignment, and test it across all models and tools using the master branch. If successful, it needs to be ported upstream to IBPSA. The motivation is to get consistency of the settings, and also because removing dp(start=dpValve_nominal/2) will lead on branch IBPSASync_issue1412_stateSelect_Tp, commit d361f82, the models Buildings.Examples.Tutorial.Boiler.System{3,4} to simulate with Optimica, which indicates that it better not be set (these models start with zero mass flow rate).

(The model Buildings.Examples.Tutorial.Boiler.System5 and higher still fail on branch IBPSASync_issue1412_stateSelect_Tp, as they do prior to removing the start value.)