Fix the i-PI communication protocol in run_driver.f90

The communication pattern in run_driver.f90 had been modified to accommodate for the needs of a specific research project, breaking completely the standard use case of communication with i-PI, or with the standard socket server in ASE.

This PR brings back the interface to a functioning state, while allowing to implement the new functionalities that had been introduced (namely the possibility of dynamically changing calculation flags, and of returning additional data on top of energy and forces).

It also adds a prominent disclaimer that future changes to run_driver.f90 should be coordinated with the i-PI developers.