add function to calculate equilibrium points

[Wxyalonso14]

add a new function in modeltools.py to calculate equilibrium points and two test_functions in test_modeltools.py

[cknoll]

Sorry for commenting not earlier. I had to fix the CI setting first, because I unintentionally broke them long time ago.

I had a first superficial look on this PR. looks good so far. However, it contains many changes to coding style, whitespaces etc. which are unrelated to the actual content. I know that the coding style is suboptimal (excuse: this is for historical reasons), but I prefer not to mix commits with new features with others "just doing cleanup".

Additionally: I renamed the default branch to "main".

Suggestion: Please file a new PR which only contains the necessary changes for the functions/methods which you added. I think the easiest way to do this is by cloning current main branch into a new directory and use copy-paste to insert your changes.

You might also file a second PR which contains changes in coding style. But probably I will do some more refactoring in the future and use an automatic tool like: https://github.com/psf/black so this work would probably be wasted time.