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 59.67%. Comparing base (eea7cf9) to head (7178a93).

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   59.66%   59.67%           
=======================================
  Files          29       29           
  Lines        4349     4364   +15     
  Branches      498      499    +1     
=======================================
+ Hits         2595     2604    +9     
- Misses       1256     1261    +5     
- Partials      498      499    +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.

[stschiff]

OK, this looks all good! Thanks. I think I'd now also like to add a feature in list to skip empty columns.