[!262] Costmodel Validation

Merge branch 'feature/costmodel-validation' into 'master'

The MR adds a new vectorization strategy target, which tries to find a
vectorization strategy whose cost (according to the model strategy) is as
close as possible to a given target. This allows to run several executables
and plot the cost model value against performance data. The actual plotting
work is done in dune-perftool-paperplots. The feature is very niche, and I
tried to minimize the occurence of code paths only necessary for this, but I
think the outcome is quite okay.

See merge request [dominic/dune-perftool!262]

  [dominic/dune-perftool!262]: gitlab.dune-project.org/dominic/dune-perftool/merge_requests/262