Added refraction-corrected altitude to planpos(31) in planet_position_la()

* Added refraction-corrected altitude to planpos(31) in planet_position_la():
  * compute properly for lcalc >= 5; set to uncorrected altitude otherwise
Speed up eq2horiz() by precomputing reused quantities

* Speed up eq2horiz() by precomputing reused quantities:
  * by ~29%
visibility: added functions mlim2skybrightness() and skybrightness2mlim()

* visibility: added functions mlim2skybrightness() and skybrightness2mlim():
  * convert back and forth:
    * naked-eye visual limiting magnitude (V)
    * sky surface brightness in (B) magnitudes per square arcsecond
Updated code_version.f90