Extend makefile

[mweidling]

When the creation of benchmarks is interrupted e.g. because of an error, the directories that are cleaned up at the end of the process (workflows/workspaces and workflows/nf-results) are still present. This causes errors in the next run of the script.

This PR adds a target that cleans said dirs before each run of make benchmarks.

[mweidling]

I messed up the git history a bit in the last week. Mea culpa. :confused:

[kba]

I messed up the git history a bit in the last week. Mea culpa. 😕

Why? Because of the merges? You can always do git log --no-merges if these trouble you. They don't trouble me.