Add Modelica Tough coupling user guide

[JayHuLBL]

No description provided.

[JayHuLBL]

@EttoreZ Would you please take another look at the changes?

[EttoreZ]

Hey @JayHuLBL just two quick comments:

• In the doStep description of the UserGuide. I would comment/remove this line: os.system("./writeincon < writeincon.inp") and uncomment this line # write_incon(), and also remove this line os.system(" ./readsave < readsave.inp > out.txt") and uncomment this line # readsave(). This is because the Python script now does not rely on the Fortran utility programs anymore.
• The second comment is also related to that. In the diagram where you show the workflow of the Python script, writeincon and readsave are cited as utility programs. I would change that to Python functions.
  Apart from those, everything else looks good to me.

[JayHuLBL]

The CI test of the TOUGH coupling example model failed to run, as it uses the Python interface function which now requires Python 3.8. However, current Dymola docker container image does not support Python 3.8, see below comments in master branch:

modelica-buildings/Buildings/Resources/Scripts/BuildingsPy/conf.yml

Line 594 in b45ef02

comment: Python 3.8 not available in docker image

We would be able to run the example models, after merging the PR #4493.

[JayHuLBL]

The terminal() function is not supported by Optimica 1.66.

The function is also being used in Buildings.DHC.Loads.HotWater.ThermostaticMixingValve. Running the model Buildings.DHC.Loads.HotWater.Examples.StorageTankWithExternalHeatExchanger with Optimica 1.66, we can see

In component theMixVal:
    The terminal() operator is not supported, and is currently evaluated to false