Added beamcentering-algorithm to processes. #133

Added functions:
 * beam_visible
 * find_beam
 * center_of_mass
 * center2beam
 * beam_centering

beam_centering encapsulates the two stages of finding the beam (find_beam), which uses beam_visible to determine when to switch to next stage, and centering (center2beam), which uses center_of_mass to determine where to move to.
Renamed center2beam to center_to_beam
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.
Included modifications made at the beamline.

Removed camera.start, .trigger, .stop for simply getting frames with grab().
Fixed bug where numpy removes quantity from pixelsize.
Added missing wait to actually wait for the motors (async) to reach the position.
Added logging in center_to_beam (and drift_to_center)
Moved center_of_mass from processes to imageprocessing
Added `@async` and Hardware-Locking for center_to_beam