Is your feature request related to a problem? Please describe.
At the moment, the freinet CSV input is really hard to read and hacky, since the functions in it have a lot of side effects.
Arrays and objects are mutated and later it is just assumed that they are correctly modified before.

Describe the solution you'd like
Use functional programming and always explicitly return the updated value without implicitly assuming the necessary modifications happened.

Describe alternatives you've considered
None.

Additional context
None.