This function is intended for use only with nestSearch.
nestSearch_step(target, j2, pix.j1)
is the target point on S^2 in spherical coordinates.
is the target resolution.
is the initial pix index, i.e., the j1-level pixel to search in.
A vector containing the HEALPix pixel index, pix
,
of the closest HEALPix pixel center to the target point,
target
, at resolution j2, and its neighbours.