Check and improve the internal documentation of every submodule

Issue

I want to verify, and if it is not the case, to abide the submodules code by this criterion :

The code is documented: each function has a doc string that describes the input and output arguments and what the function does, and each Jupyter cell has a short comment that describes the purpose of the cell (doc strings, headers)

Goals

• Realize issue #17 (closed).
• Realize issue #16 (closed).
• Realize issue #15 (closed).
• Realize issue #14 (closed).
• Realize issue #31 (closed).
• Realize issue #32 (closed).