remove completely the energy concern from the core

- It is moved to the plugin only.
- In particular, there is nothing in the core headers anymore
- SMPI still defines some energy methods
- the command-line config still needs to know the init method

Also, I'm sorry, but I renamed simgrid::Host->(p_cpu/p_netcard) to
pimpl_cpu/pimpl_netcard. I should have separated the rename, sorry.