This function calculates the near and far kinematic distances for a
given galactic longitude and radial velocity. The calculation uses
the Galactic rotaion model of Brand and Blitz 1993, A&A, 275 : 67.
result=KDIST( longitude, velocity, [/RADIANS])
longitude: Galactic Longitude. Currently must be in the range [-180,
180] in degrees.
velocity: Radial velocity in km/s
RADIANS: If set, input Longitude is in radians
DEBUG: If set, produce debugging plots / information
The two element vector [near_distance, far_distance] in kpc.
Currently only computes distances for objects in the inner galaxy.
Written by: Chris Beaumont, June 2008.
June 23, 2008: Changed name from kinematic_distance to kdist. cnb
June 23, 2008: Fixed bug in modding l with 2 pi. cnb
July 17, 2008: Removed degrees keyword. Added radians. cnb
July 17, 2008: Changed things so that, if l is a variable,
it isn't modified
March 18, 2009: Changed theta_0 and v_sol to reflect arxiv
March 20, 2009: Changed minor typo in value of a1. Added /DEBUG keyword