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.