[RTR] Preparing the work for refactoring external forces

[Ipuch]

• [x] Linked issue related to your pull request using :

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• [x] Used the tag [WIP] or [RTR] for on-going changes, or for ready to review
• [x] Did you write new tests to cover your changes?
• [x] Did you write a proper documentation (docstrings and ReadMe)
• [x] Please black your code black . -l120

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This change is 

[codeclimate[bot]]

Code Climate has analyzed commit 31219df9 and detected 4 issues on this pull request.

Here's the issue category breakdown:

Category	Count
Complexity	4

View more on Code Climate.