Introduce box plot drawing script

[serpent7776]

Description

This adds a script that is drawing a box plot from the BENCHMARK measurement data. It is implemented in R using XML and ggplot2 libraries. Xml reporter has been adjusted to include the measurement data.

Each TEST_CASE is drawn as a single plot and then exported as a png file named after the test case, with spaces replaced with underscore.
All plots are also saved to a single file Rplots.pdf.
Tests that don't have any measurement data are skipped.

GitHub Issues

N/A

Example plots from existing benchmark tests:

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 90.95%. Comparing base (fa5a53d) to head (95fa9b2).

Additional details and impacted files

@@            Coverage Diff             @@
##            devel    #2869      +/-   ##
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- Coverage   90.99%   90.95%   -0.04%     
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  Files         198      198              
  Lines        8391     8395       +4     
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  Hits         7635     7635              
- Misses        756      760       +4