NAME:
find_closest_trackpointLB
INPUT:
l,b,D,vlos,pmll,pmbb- coordinates in (deg,deg,kpc,km/s,mas/yr,mas/yr)
interp= (True) if True, return the closest index on the interpolated track
usev= (False) if True, also use the velocity components (default is to only use the positions)
OUTPUT:
index of closest track point on the interpolated or not-interpolated track
HISTORY:
2013-12-17- Written - Bovy (IAS)