Add stabsel directionality plot 2

[machlabd]

added new plot function for stability selection. One thing that's crossed my mind is that with all these functions it would look much neater if I had one summarized experiment object instead of requesting so many parameters for every plotting function (all that's related to stability selection part). I've so far stuck to this so that i have a 'stabs' object (since we use that package to do stability selection) but maybe it's worth rethinking or restructuring after this merge.

[codecov[bot]]

Codecov Report

Merging #31 into master will increase coverage by 0.16%. The diff coverage is 95.12%.

@@            Coverage Diff             @@
##           master      #31      +/-   ##
==========================================
+ Coverage    90.2%   90.36%   +0.16%     
==========================================
  Files           8        8              
  Lines        1082     1121      +39     
==========================================
+ Hits          976     1013      +37     
- Misses        106      108       +2

Impacted Files	Coverage Δ
R/plotting.R	99.11% <95.12%> (-0.89%)	:arrow_down:

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[mbstadler]

I agree with your comment about the SE as a return value also for stabsel. It would make things more consistent with the other parts of monaLisa. Feel free to open an issue if like.

[mbstadler]

Do I understand correctly that the branch add_plot_for_stabsel is not needed anymore and can be deleted?