Function htmIndex( level, lon, lat )

Description:
Gives the HTM (Hierachical Triangular Mesh) pixel index for a given sky position.
Parameters:
level (integer)
HTM level
lon (floating point)
longitude in degrees
lat (floating point)
latitude in degrees
Return Value (long integer):
pixel index
See Also:
HTM web site
Signature:
long htmIndex(int, double, double)