Refactoring of the Rapl backend

This is a follow-up of !104 (merged) where the code is simplified/cleaned-up using a new RaplCounter class.

The common/io.h file has been removed, in favour of using std::ifstream to read from files.

The Rapl backend is now also aware of what CPUs are used, so that in case of multi-socket systems, only the active sockets are measured.