Allow to use only subset of all available cores

[erthalion]

Currently workload is spread across all available cores:

berserker/src/main.rs

Line 49 in 73d8499

let handles: Vec<_> = iproduct!(core_ids.into_iter(), 0..9)

For certain future use cases it has to be more flexible. Allow to configure a range of CPU cores to run on.