Costmodel Validation

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.