benchmark

Introduce warmup phase to BenchmarkRunner (#1130) (#1399)

* Introduce warmup phase to BenchmarkRunner (#1130)

In order to account for caching effects in user
benchmarks introduce a new command line option
"--benchmark_min_warmup_time"
which allows to specify an amount of time for
which the benchmark should be run before results
are meaningful.

* Adapt review suggestions regarding introduction of warmup phase (#1130)

* Fix BM_CHECK call in MinWarmUpTime (#1130)

* Fix comment on requirements of MinWarmUpTime (#1130)

* Add basic description of warmup phase mechanism to user guide (#1130)