PlannerBenchmark should be able to return the benchmark report directory (API gap)

*benchmark directory Unable to render embedded object: File (= benchmark report directory* (and yes, these probably should get better names - now is the time as this PR will expose more into the API) not found.)

A PlannerBenchmarkFactory has a benchmarkDirectory, for example "local/vrp"
A PlannerBenchmark has a benchmarkReportDirectory for example "local/vrp/2020-08-20_160433"

Users need to be able to get that 2014-08-20_160433, because they can't set it (nor they want to) and they currently can't get it.