returns the Parallactic angle given hour angle (hrs), dec (degs), and latitude (degs)
solve spherical triangle using Napier's analogies
the angle A and opposing side a correspond to ra1,dec1
the angle B and opposing side b correspond to ra2,dec2
the angle C and opposing side c correspond to the north celestial pole
Code due to Greg Aldering, aldering@mh1.lbl.gov