add $addPlots() method to PlotGridConfiguration

[IndrajeetPatil]

Closes #268

[codecov-commenter]

Codecov Report

Merging #269 (24a4415) into develop (486187b) will increase coverage by 0.14%.
The diff coverage is 100.00%.

❗ Current head 24a4415 differs from pull request most recent head 468df4d. Consider uploading reports for the commit 468df4d to get more accurate results

@@             Coverage Diff             @@
##           develop     #269      +/-   ##
===========================================
+ Coverage    51.25%   51.39%   +0.14%     
===========================================
  Files           51       51              
  Lines         2790     2796       +6     
===========================================
+ Hits          1430     1437       +7     
+ Misses        1360     1359       -1     

Impacted Files	Coverage Δ
R/plot-grid.R	30.76% <100.00%> (+25.76%)	⬆️

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