Improve test coverage, fix remove_slicing, remove mul_skipZeros! of VCAT

[hakkelt]

Things get fixed:

• GitHub actions start Julia with a single thread by default, but it disabled many tests for multithreaded branches. This got fixed in ci.yaml
• VCAT and Compose had low code coverage, so I extended the tests. During that time, I discovered some issues with remove_slicing that I fixed.
• I removed the non-exported function mul_skipZeros! because it is not covered with tests and does not appear to be used anywhere (including StructuredOptimization.jl).
• BroadCast and GetIndex received minor fixes, and the latter was also formatted.

[codecov[bot]]

Codecov Report

:x: Patch coverage is 94.52055% with 8 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 85.89%. Comparing base (c8e2903) to head (c1d3de1). :warning: Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/linearoperators/GetIndex.jl	96.03%	4 Missing :warning:
src/utils.jl	83.33%	2 Missing :warning:
src/calculus/Compose.jl	87.50%	1 Missing :warning:
src/calculus/VCAT.jl	94.73%	1 Missing :warning:

Additional details and impacted files

@@             Coverage Diff             @@
##           master      #28       +/-   ##
===========================================
+ Coverage   72.66%   85.89%   +13.22%     
===========================================
  Files          44       45        +1     
  Lines        3077     3063       -14     
===========================================
+ Hits         2236     2631      +395     
+ Misses        841      432      -409     

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[hakkelt]

This PR is also ready for merging. @nantonel @lostella: Can you confirm that mul_skipZeros! from VCAT is safe to remove?