Allow to set single values on percpu maps with python

I'm using a percpu_hash and I need to modify the value on a map specific to a cpu from the userland.

As introduced with the PR #458 the map can be modified via the python binding but the entry of all maps has to be modified at the same time.
This is something I need to avoid since I don't want to modify the entries of maps belonging to other CPUs.

Currently I'm doing something like:
```
global_value = bpf_map[key]
global_value[cpu] = 0
bpf_map[key] = global_value
```
However between the read at line 1 and the write at line 3 the value of the maps not belonging to `cpu` and copied in `global_value` could have been changed.

Is there a way to only update the map belonging to `cpu`?

Review PR #458 and the Python binding entry point for percpu_hash map access first. Trace how bpf_map[key] reads and writes all CPU values, then identify the binding and tests that cover per-CPU updates. Done means userland can update only the selected CPU's map entry without overwriting concurrent changes on other CPUs.

Written by the indexing model from the issue text.