Plotting test case improvements

Various improvements to plotting test case:

• Plotting artifacts are now all in one folder.
• Artifacts are not uploaded if the identical artifacts are already in the parent commit. A README file tells the user where to find the plots.
• By default, the plotting is compared to the git merge-base. If on gitlab the environment variable referenceCommit_$branchName (where $branchName is the name of the git branch, in which occurrences of - are replaced by _) is set to a commit hash, then this commit is taken as reference.
• Fix small inconsistencies with test tags.

Closes issue #141 (closed).