Completely unrealistic beatnote frequencies for 'six' laser locking configuration

At the moment, all carrier offsets are set to 0.0 if 6 free-running lasers are used. This has as a consequence that most beatnote frequencies are very unrealistically small in that setting, making some noise couplings appear smaller than they'll be in reality. There is no way to manually override this setting at the moment, as the offset values are hard-coded in the function lock_on_cavity, which is called after the simulation has started.

A solution could be to add another parameter to init, which can be either a single double offsetting all laser frequencies in the case locking is used, or a dictionary of individual offsets if lock='six' is used.