Switch group example (Marker API)

[carstenbauer]

As far as I can see, there is no group switch in the example code at https://juliaperf.github.io/LIKWID.jl/dev/examples/perfmon/, that's why only the first group gets measured (| is the group delimiter). You would need to run your benchmark code twice, once for the first group, then "switch groups" and run your benchmark again for the second group measurement.
The two groups have different gids, so when reading the results/metrics, you have to specify the right gid.

Originally posted by @TomTheBear in #28 (comment)

[carstenbauer]

Note that one can also already measure different groups with @perfmon / perfmon, i.e. without using the Marker API.