visibility.f90: updated procedure descriptions
New function aperture() to compute aperture needed to observe an object

* New function aperture() to compute aperture needed to observe an object:
  * as a function of excess magnitude, and optionally pupil diameter (mm)
  * returns aperture diameter in cm
limmag_full(): added humidity, temperature and vision as optional arguments

* limmag_full(): added humidity, temperature and vision as optional arguments:
  * default values: rh=50%, T=10dC, sn=1
Changed default relative humidity 50%->70%, since this is at night