Simplify mean and std computation

Closes #100 (closed)

changed milestone to %DAN-P4: Improve data loading and preprocessing

added P2 label

assigned to @melodie.boillet

I didn't use the same computation as in Doc-UFCN (https://gitlab.com/teklia/dla/doc-ufcn/-/blob/main/doc_ufcn/train/normalization_params.py#L20) since it's not the same average (micro vs macro). I kept the original average but simplified a bit the code

requested review from @starride

requested review from @schneider-y and removed review request for @starride

resolved all threads

approved this merge request

merged