Returns the longitude of the approximate center of the tile
with a given index in the HEALPix RING scheme.
Note: the index parameter is 0-based,
unlike the table row index special token $index
(a.k.a. $0), which is 1-based.
So if the HEALpix index is implicitly determined by the table row,
the value $index-1 should be used.