Added a feature to remove empty .janno columns with rectify

[nevrome]

I started to think about #298 and realized rectify doesn't touch .janno files at all so far. So I thought I first add a simpler .janno-editing feature: Removing empty columns. That is also a pretty common task. Maybe one day rectify will rather be modify 🤔

[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 8 lines in your changes missing coverage. Please review.

Project coverage is 60.42%. Comparing base (d918c11) to head (f2cc266).

Files with missing lines	Patch %	Lines
src/Poseidon/CLI/Rectify.hs	60.00%	3 Missing and 1 partial ⚠️
src/Poseidon/CLI/OptparseApplicativeParsers.hs	0.00%	3 Missing ⚠️
src/Poseidon/Janno.hs	94.11%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #326      +/-   ##
==========================================
- Coverage   60.43%   60.42%   -0.01%     
==========================================
  Files          29       29              
  Lines        4218     4233      +15     
  Branches      489      490       +1     
==========================================
+ Hits         2549     2558       +9     
- Misses       1180     1185       +5     
- Partials      489      490       +1     

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[stschiff]

Very nice. I haven't tested it, but the unit tests look convincing. I left a few comments about the empty-string to "n/a" feature.

[nevrome]

I found a better solution for explicitNA. This operates more upstream, thus also affecting the output of list. Please have another look.