Issue1890 nominal start values

[mwetter]

This closes #1890

[jelgerjansen]

@mwetter looks good! I did a quick check by compiling IBPSA.Examples.Tutorial.SimpleHouse.SimpleHouse6. It appears that the start value of Modelica.Units.SI.ThermodyanmicTemperature and Modelica.Media.Interfaces.Types.MassFraction gives rise to quite some warnings, but I get that it won't be possible to change these.

However, I also noticed warnings similar to this one:

[16] 10:46:24 Translation Warning
Alias set with conflicting start values
 * Candidate: rad.port_b.h_outflow(start = 83680.0, confidence number = 8)
 * Candidate: rad.vol[5].dynBal.medium.h(start = rad.vol[5].dynBal.hStart = IBPSA.Examples.Tutorial.SimpleHouse.SimpleHouse6.rad.vol.dynBal.Medium.specificEnthalpy_pTX(300000.0, 293.15, {1.0}), confidence number = 4)
 * Candidate: rad.vol[5].dynBal.hOut(start = rad.vol[5].dynBal.hStart = IBPSA.Examples.Tutorial.SimpleHouse.SimpleHouse6.rad.vol.dynBal.Medium.specificEnthalpy_pTX(300000.0, 293.15, {1.0}), confidence number = 4)
=> Select value from rad.vol[5].dynBal.hOut(start = rad.vol[5].dynBal.hStart = IBPSA.Examples.Tutorial.SimpleHouse.SimpleHouse6.rad.vol.dynBal.Medium.specificEnthalpy_pTX(300000.0, 293.15, {1.0})) for variable: rad.vol[5].ports[2].h_outflow

Would it make sense to change hStart in IBPSA.Fluid.Interfaces.ConservationEquation (lines 156-160) to Medium.h_default? Or do you prefer to keep using the function Medium.specificEnthalpy_pTX(p_start, T_start, X_start) as this is a more general (and safe) option?

[mwetter]

@jelgerjansen : hStart should be left as is, e.g., its value should be based on {p,T,X}_start as it is the initial value used for the enthalpy that need to be consistent with p,T,X.