[feature] Make preprocessing more modular

[WilliamCappelletti]

Is your feature request related to a problem? Please describe.

Currently much of the preprocessing is duplicated (e.g. channel validation and resampling). Furthermore, there is no option to add custom steps (e.g. bandpass filtering or Fourier transforms)

Describe the solution you'd like

• Keep a module that loads the data from EDF or dataset-specific format into a uniform dataframe.
• Centralize the (optional) resampling
• Allow for user-defined processing (matrix-to-matrix)

Describe alternatives you've considered

We could apply user-defined transform in a second moment by iterating over all files, but it might be problematic if the process is not completed in one pass.