Moved procedures to new file visibility.f90

* Moved procedures to new file visibility.f90:
  * procedures to determine the visibility of objects
  * from functions.f90
  * best_planet_visibility(), planet_visibility_tonight(), comet_invisible(),
    transitalt(), best_obs_date_ra(), get_dRA_obj()
  * updated CMake files
New procedures calc_gmst(), airmass() and airmass_ext()

* New procedures calc_gmst(), airmass() and airmass_ext():
  * moved from libSUFR, astro module
  * libTheSky is more appropriate