pix.HPDataFrame: HEALPix pixel indices from HPDataFrame
Description
If new.pix is unspecified then this function returns the vector of
HEALPix pixel indices from a HPDataFrame. If new.pix is specified then
this function returns a new HPDataFrame with the same number of rows
as x, but with pix attribute new.pix. Thus,
new.pix must have length equal to nrow(x).