1133 Change member functions in Model class from public to private in IDE model

[annawendler]

Changes and Information

• Make member functions that are required for simulating private
• Define Simulation class and set_initial_flows() as friends

Merge Request - Guideline Checklist

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Checks by code author

• Every addressed issue is linked (use the "Closes #ISSUE" keyword below)
• New code adheres to coding guidelines
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• Tests are added for new functionality and a local test run was successful (with and without OpenMP)
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Checks by code reviewer(s)

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Closes #1133

[lenaploetzke]

Nice PR, I guess this really prevents the model functions from being misused. :) I just have a few comments. Please also tick the boxes of the checks by author.