sitd - compute spherical-head model ITD.
itd = sitd( az, el, hr, sld )
itd - ITD specified as a left or right ear lag relative to the opposite ear,
positive = lag left ear, negative = lag right ear, us
az - azimuth, degrees
el - elevation, degrees
hr - head radius, m
sld - source-listener distance, m
This function is based on a similar function from the paper:
Miller, Joel D., "Modeling Interaural Time Difference Assuming a Spherical
Head", Music 150, Musical Acoustics, Stanford University, December 2, 2001.
http://www.sonisphere.com/SLAB/MUS150Paper.pdf
The author has allowed this function to be included in the slab3d release.