Paradigm for "cropped" epoching

[PierreGtch]

Implementing a paradigm that allows doing a "cropped" epoching (in the sense of braindecode: https://braindecode.org/stable/auto_examples/plot_bcic_iv_2a_moabb_cropped.html) would be relatively simple with the new modular paradigms. We would just need to implement a new processing step that computes events from raw objects (i.e. similar to RawToEvents and RawToFixedIntervalEvents).