Add example on ERP

[qbarthelemy]

This PR:

• adds plot_erp and complete test;
• add an example on ERP display, especially with 2D histogram.

It also renames the ERP example on embedding, because the script selects MEG channels instead of EEG ones. @plcrodrigues

[plcrodrigues]

I like this new functionality quite a lot. Thanks, @qbarthelemy

I will however play the devil's advocate and ask the following question: Should we really use the term Event-related potential ?. I know this is the correct terminology when we are dealing with EEG-related applications and neuroscience. However, what if we called it something like a "multi-trial" signal ? This would make the package a bit more agnostic regarding its applications on signal processing. What do you think ?

[sylvchev]

I like this new functionality quite a lot. Thanks, @qbarthelemy

Same here! Thanks @qbarthelemy

I will however play the devil's advocate and ask the following question: Should we really use the term Event-related potential ?. I know this is the correct terminology when we are dealing with EEG-related applications and neuroscience. However, what if we called it something like a "multi-trial" signal ? This would make the package a bit more agnostic regarding its applications on signal processing. What do you think ?

I agree on the general line of questioning, but for this example I don't have a specific term in mind for the general signal processing case, whereas ERP is quite specific of this situation. Thus I'm in favor of keep the plot_erp name.

[agramfort]

why having both options? is it really necessary?

…

[qbarthelemy]

This would make the package a bit more agnostic regarding its applications on signal processing. What do you think ?

I totally agree. With this function, we could plot QRS peaks from ECG.

I have updated documentation (ERP has been replaced by waveform), but also function name (otherwise, erp term is unexplained).

[qbarthelemy]

why having both options? is it really necessary?

Ok, I will refactor the function to remove this parameter.

[qbarthelemy]

@sylvchev , any idea about documentation build fail?
https://github.com/pyRiemann/pyRiemann/runs/4109284600?check_suite_focus=true#step:4:118

EDIT: solved defining mne==0.23.4 in doc/requirements.txt (problem in 0.24.0)

[agramfort]

thx @qbarthelemy !