MRG: Refactor EDF channel picking

[larsoner]

@cbrnr to tackle #1766 I first want to refactor the EDF reader to hold a "sel" of the channels to be used. Can you see if this breaks anything with your set of EDF data files? Tests are passing at least.

[cbrnr]

All good. However, note that if there is a TAL channel, data must be preloaded (otherwise you get this RuntimeError: EDF+ Annotations (TAL) channel needs to be parsed completely on loading. You must set preload parameter to True.). I'm not sure if this is relevant for what you want to do.

[codecov]

Codecov Report

Merging #5137 into master will decrease coverage by <.01%.
The diff coverage is 98.07%.

@@            Coverage Diff             @@
##           master    #5137      +/-   ##
==========================================
- Coverage   88.14%   88.14%   -0.01%     
==========================================
  Files         357      357              
  Lines       65777    65765      -12     
  Branches    11192    11185       -7     
==========================================
- Hits        57982    57970      -12     
  Misses       4958     4958              
  Partials     2837     2837

[larsoner]

It won't really be relevant fortunately

[agramfort]

@cbrnr feel free to merge then.

[cbrnr]

@agramfort I don't have the permission to merge, but you guys go ahead, my test files can still be loaded!

[agramfort]

@cbrnr now you should be able to merge :)

[cbrnr]

Thanks @agramfort, I'm happy to be part of the team! Thanks @larsoner, PR merged.