Allow notch filtering of epochs

[ikarabinas]

Reference issue

Resolves #12398

What does this implement/fix?

Moved notch_filter from mne/io/base.py to class FilterMixin in mne/filter.py to allow filtering of Epoch objects, not only Raw. Fixed broken link to the contributing guidelines.

[welcome]

Hello! 👋 Thanks for opening your first pull request here! ❤️ We will try to get back to you soon. 🚴

[larsoner]

Looks like a good start! Can you look at why the CIs fail? They should tell you some stuf about what is wrong. If it doesn't make sense I can give more pointers (but I'm hoping they're maybe clear enough!).

Any new change should come with updated tests that ideally fail on main but pass on the new PR. In your case that would probably involve writing some code that calls epochs.notch_filter -- that wouldn't exist on main (and hence fail) but should pass on this PR. I'd suggest around here in test_epochs.py somewhere since it's where we test other Epochs filtering methods:

mne-python/mne/tests/test_epochs.py

Line 1044 in 87df00d

assert_allclose(epochs.get_data(), data_filt, atol=1e-17)

It would be awesome if you computed the PSD before and after and checked for example that you could remove 60 Hz noise -- but even starting out with having a call to epochs.notch_filter(...) would be a good start!

[larsoner]

This one should actually still point to the dev page because it stays most up-to-date (updated roughly daily), and so is the best guide for current practice

Suggested change

	The [contributing guide](https://mne.tools/stable/development/contributing.html) has details on the preferred contribution workflow
	The [contributing guide](https://mne.tools/dev/development/contributing.html) has details on the preferred contribution workflow