MRG: Add test that NIRS naming matches stored frequency

[rob-luke]

Reference issue

General fNIRS tweaks #7057.

What does this implement/fix?

In #7064 the NIRS light frequency was moved from being stored in the channel name to being stored in info['chs'][ii]['loc'][9]. However, the frequency is still stored in the channel name for ease of reading, plotting, and because MNE throws an error if two channels have the same name.

This PR adds a check that the light frequency stored in the ch_name and in loc match. And adds tests.

Additional information

People shouldn't mess with the channel names and this is specified in the docs already. https://github.com/mne-tools/mne-python/blob/master/tutorials/io/plot_30_reading_fnirs_data.py#L26

[larsoner]

Looks like reasonable sanity checking, +1 for merge if you're done @rob-luke

[rob-luke]

Just fixed a typo. But I am done now.

[rob-luke]

@larsoner I always prefer more checks, but I guess these things slow the code down. Whats the general preference around here?

[larsoner]

I guess these things slow the code down. Whats the general preference around here?

In general more checks are better. Typically we have slowdowns in the hundreds of ms to seconds range (even plotting is here), so something like this that adds what I assume is < 1 ms is fine.

[larsoner]

I should say typically we have these slowdowns when we want to do something interesting, not in checks like this...

[rob-luke]

In general more checks are better.

Thanks.

circle is failing, but looks unrelated

[larsoner]

The 3.7 pip build on Travis is probably also going to fail, both hopefully will be fixed by #7501

[codecov]

Codecov Report

Merging #7499 into master will decrease coverage by 2.44%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #7499      +/-   ##
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- Coverage   90.12%   87.67%   -2.45%     
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  Files         453      453              
  Lines       82661    82666       +5     
  Branches    13052    13053       +1     
==========================================
- Hits        74496    72478    -2018     
- Misses       5344     7390    +2046     
+ Partials     2821     2798      -23

[agramfort]

thx @rob-luke