New subroutine best_planet_xsmag() to compute moment of best excess magnitude for a planet

* New subroutine best_planet_xsmag() to compute moment of best excess magnitude for a planet:
  * input: JD and planet ID
  * output: best JD and excess magnitude (magnitude - limiting magnitude)

  * renamed excsmag -> xsmag in procedure and variable names
Made dummy variables in planet_visibility_tonight() optional

* Made dummy variables in planet_visibility_tonight() optional:
  * rts, tts, sts, tas are often not required
  * made optional, and moved to the back of the dummy-argument list