Cleanup Special Matrices code

[alyst]

Simplifies a bit the code to generate elimination, duplication and commutation matrices.
The latter is implemented as a special CommutationMatrix <: AbstractMatrix{Int} subtype, which provides elegant integration into LinearAlgebra API.

I was actually trying to check, whether there are Julia packages implementing these special matrices.
So far I have found VectorTransformations.jl package, but it works by constructing sparse matrices with the predefined structure, not by implementing custom code that does the expected transformation directly (which might be faster).

Cherry-picked from #193

[Maximilian-Stefan-Ernst]

Nice, this is ready to merge once the tests pass. I have added some unit tests for the special matrices.

[codecov]

Codecov Report

Attention: Patch coverage is 89.74359% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 69.54%. Comparing base (d9884ae) to head (3abe92e).
Report is 3 commits behind head on devel.

❗ Current head 3abe92e differs from pull request most recent head c1e7a69. Consider uploading reports for the commit c1e7a69 to get more accurate results

Files	Patch %	Lines
src/additional_functions/commutation_matrix.jl	85.71%	3 Missing ⚠️
src/additional_functions/helper.jl	93.75%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##            devel     #202      +/-   ##
==========================================
+ Coverage   67.86%   69.54%   +1.68%     
==========================================
  Files          51       52       +1     
  Lines        2483     2423      -60     
==========================================
  Hits         1685     1685              
+ Misses        798      738      -60     

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[Maximilian-Stefan-Ernst]

Thanks for the additional tests. Should we merge now? It seems like formatter still complains...

[alyst]

It looks ready to me.
I have no idea why the formatter complains: the diff looks identical, I don't see any trailing spaces or anything like that. But if there would be any issues upon merging, should be easy to fix.

[alyst]

Thank you for reviewing and merging! I'm preparing a next portion of commits to cherry-pick, but it would have to wait a day or two.