New functions for limiting magnitude

* New functions for limiting magnitude:
  * limmag_sunmoon():  full function, dependent on date, location, Sun alt/elon,
    Moon phase/alt/elon, object altitude
  * limmag():  strongly simplyfied, dependent on Sun altitude only(!)
New functions to compute the excess magnitude of a planet

* New functions to compute the excess magnitude of a planet:
  * functions pl_excess_magn(): full function
  * functions pl_excess_magn_pl(): version for solvers

  * Excess magnitude:
    * just made it up
    * difference between  object magnitude, corrected for airmass  and  limiting magnitude
    * <0 if (theoretically) visible with the naked eye
    * very simplified, don't trust magnitudes too much, but can trust best moment,
      i.e. moment of lowest excess magnitude
New wrapper routines for limiting magnitude

* New wrapper routines for limiting magnitude:
  * renamed function limmag_sunmoon() -> function limmag_full() - full routine
  * renamed function limmag() -> function limmag_sun() - uses Sun altitude only

  * added wrapper function limmag_jd():
    * calls limmag_full() with self-computed data for Sun and Moon
    * provide JD and object RA, dec, alt

  * added wrapper function limmag_jd_pl():
    * calls limmag_jd() with self-computed data for a given planet
    * provide JD and planet ID
Visibility: use low-accuracy routines for planet positions
Visibility: added full function to compute the excess magnitude

* Visibility: added full function to compute the excess magnitude:
  * uses low-accuracy routines for planet positions
  * uses full version for limiting magnitude