cnt: use set_montage at the end of the __init__

[massich]

This PR is part of #6461. The concrete goal here is to make sure that:
1 - RawCNT is always called with montage=None
2 - ensure self.set_montage(montage) is called at the end of init

[codecov]

Codecov Report

Merging #6471 into master will increase coverage by 0.03%.
The diff coverage is 90.9%.

@@            Coverage Diff             @@
##           master    #6471      +/-   ##
==========================================
+ Coverage   89.21%   89.25%   +0.03%     
==========================================
  Files         413      413              
  Lines       74286    74338      +52     
  Branches    12277    12282       +5     
==========================================
+ Hits        66277    66348      +71     
+ Misses       5148     5134      -14     
+ Partials     2861     2856       -5

[larsoner]

I don't know of any CNT montage. You can just create one from scratch using the correct ch_names and np.random.randn(n_chs, 3) if you want.

[agramfort]

thx @massich