Compute the horizontal parallax using constants from libSUFR

* Compute the horizontal parallax using constants from libSUFR:
  * from earthr and au
  * rather than ad-hoc
sun: low-accuracy routine gives better results when using UT

* sun: low-accuracy routine gives better results when using UT:
  * 10000 random positions for 2015-2014, compared to VSOP87
    * use JD iso JDE
    * accuracy in ecliptic lon.:  13.759 +- 6.991 "  ->  12.082 +- 5.824"
    * in fact, compares even better when using JD - DeltaT/86400 (!)
Fixed Doxygen comment
Use a (supposedly) more accurate routine to compute airmass

* Use a (supposedly) more accurate routine to compute airmass:
  * replaced airmass()
    * Young (1994);  maximum supposed error (at the horizon) of 0.0037 air mass
  * old routine now called airmass2()  (Kasten and Young, 1989)

  * added some references as comments
sun: added comments and commented-out alternatives; compute topocentric alt for calc>=99

* sun: added comments and commented-out alternatives
  * info on stand-alone use (how to provide and get input using modules)
* compute topocentric alt for calc>=99
  * uncorrected (30) and corrected (31) for atmospheric refraction