Add Modelica Tough coupling user guide

[JayHuLBL]

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[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]

@EttoreZ Thanks for the comments. I just addressed it. Please double-check.

[EttoreZ]

@JayHuLBL the changes look good to me.