Use libSUFR's deq() and dne() to test (non-)equalities of floating-point numbers
Added function comet_invisible() to determine whether a comet is invisible

* Added function comet_invisible() to determine whether a comet is invisible:
  * this is meant as a cheap routine to quicly cut out comets that should not be considered
  * if .true., comet may still be invisible (e.g. close to the Sun)
  * based on JD, magnitude limit and transit altitude limit