A detailed guide on preprocessing

For ephys pipelines there are a number of quite complex preprocessing steps that need to be done in a specific order. Also, there are some nuances to some preprocessing steps, particulaly around multi-shank and bad channel detection. For example CAR, IBL bad channel detection*, highpass spatial filter etc make assumptions on channel location that are broken in the multi-shank contex. Bad channel detection should not be done after CAR etc.

In general I think these preprocessing steps are all quite mysterious if you are not familiar with them. I've been meaning for ages to write a detailed guide on preprocessing steps (what and why for each step) but never gotten around to it, and speaking to people at the conference I think it would be useful. So I'll post this issue to get the ball rolling and invite contributions, maybe opening a PR next week!

*BTW I just realised this yesterday for IBL bad channel detection, maybe we should add an assert / warning?
*also whitening, potentially.

No file or test entry point is named. Start by mapping the preprocessing steps mentioned—CAR, IBL bad-channel detection, highpass spatial filtering, and whitening—and documenting their required order, rationale, and multi-shank caveats. Done should be a detailed guide explaining what and why for each step, including the channel-location assumptions and bad-channel timing concerns.