v1.12.0

• Require sgp4>=2.3
• Use mean Earth radius for illumination computation
• Use current date if not given in get_only_position too
• Document mean_motion (finally!)
• Add R_E_MEAN_KM, ALPHA_UMB and ALPHA_PEN constants
• Add period property to predictors
• Add get_shadow, get_eclipse_duration, get_beta, get_normal_vector
  and eclipses_since methods to predictors
• Make sate_id mandatory and source optional