Show batch time comparisons in `nvbench_compare.py`

[bernhardmgruber]

When comparing benchmark results with nvbench_compare.py, currently, the cold measurements are compared and shown. If a JSON file also contains batch measurements, those should be shown in addition.

[ahendriksen]

The batch timings are often ignored, see also #194 . Perhaps it is possible to introduce an abstraction that makes it easy to always grab the "best" timing, depending on whether batch or non-batch timing is available.