Renamed center_to_beam (and center2beam from line 537) to drift_to_beam.

#133
Renamed it because the way of iterating towards the beam seems more like drifting and I want to use the name center_to_beam for what is currently called beam_centering.
Renamed beam_centering to center_to_beam
Moved beam_visible into find_beam.

Since like sensej said it's specific for find_beam. But I keep it in a seperate function for easy changing in the future if needed.
Removed abort-argument.

Its useless in its current state. I will look at it after issue #159 is merged.