avoid overall comparison with old versions until we have comparables

Bureaucrats is currently the first model to run, but we don't have the
old benchmark data for it, so the script was trying to get a mean from
an empty list of comparisons and crashing on reduceLeft
only run until we have one good result for each model
write benchmark results to tmp/bench.txt
Merge branch '5.0.x' into benches-improvements
Revert "only run until we have one good result for each model"

This reverts commit 96b085a701fb707913b1fc28e0f00db45e3030e9.
Merge pull request #403 from NetLogo/benches-improvements

Benchmark script improvements