Polishing low-accuracy routines for Sun and Moon

* Polishing low-accuracy routines for Sun and Moon:
  * ensure planpos = 0 at start of planet_position_la()
  * add apparent diameter in planet_position_la()
  * remove filling of topocentric variables with geocentric values from moonpos_la(),
    as this now happens in planet_position_la()
    * this fixes a BUG  (typo:  planpos(29) = hh  should be  planpos(28) = hh)
Fill out more values in planpos when calling planet_position_la() for Sun, Moon

* Fill out more values in planpos when calling planet_position_la() for Sun, Moon:
  * planpos(7, 11-17, 41-43)
Moved Sun procedures to new file sun.f90;  moved la Moon code to moon_position.f90

* Moved Sun procedures to new file sun.f90:
  * to avoid circular dependencies
  * sun.f90 contains subroutine sunpos_la() and function sunmagn()

* Moved low-accuracy Moon code to moon_position.f90:
  * from planet_position_la() to moonpos_la()
  * caused circular dependencies, hence new file sun.f90

* Updated CMake files
planets: make arguments calc,nt in subroutine planet_position_la() optional

* planets: make arguments calc,nt in subroutine planet_position_la() optional:
  * for similarity with planet_position()
  * default values: compute as much as possible and as accurately as possible