Make a SkyWcs object with HEALPix grid projection (HPX).
The SkyWcs generated by this function is suitable to be used with a
Hierarchical Progressive Survey (HiPS) FITS image as described in
https://www.ivoa.net/documents/HiPS/20170519/REC-HIPS-1.0-20170519.pdf
A HiPS image covers one HEALPix cell, with the HEALPix nside equal to
2**hips_order. Each cell is 'shift_order' orders deeper than the HEALPix
cell, with 2**shift_order x 2**shift_order sub-pixels on a side, which
defines the target resolution of the HiPS image. The IVOA recommends
shift_order=9, for 2**9=512 pixels on a side. See Notes below to
convert from hips_order to image resolution.
Parameters
----------
hips_order : `int`
HiPS order, such that HEALPix nside=2**hips_order.
Must be a positive integer.
pixel : `int`
Pixel number in the nest ordering scheme.
shift_order : `int`, optional
Shift order for subpixels, such that there are 2**shift_order
sub-pixels on a side of the HiPS cell.
Must be a positive integer.
Returns
-------
wcs : `lsst.geom.SkyWcs`
Raises
------
`ValueError`: Raise if hips_order is <=0, or if shift_order is <=0, or
if pixel number is out of range for the given hips_order
(0 <= pixel < 12*nside*nside).
Notes
-----
Table 5 from
https://www.ivoa.net/documents/HiPS/20170519/REC-HIPS-1.0-20170519.pdf
shows the relationship between hips_order, number of tiles (full
sky coverage), cell size, and sub-pixel size/image resolution (with
the default shift_order=9):
+------------+-----------------+--------------+------------------+
| hips_order | Number of Tiles | Cell Size | Image Resolution |
+============+=================+==============+==================+
| 0 | 12 | 58.63 deg | 6.871 arcmin |
| 1 | 48 | 29.32 deg | 3.435 arcmin |
| 2 | 192 | 14.66 deg | 1.718 arcmin |
| 3 | 768 | 7.329 deg | 51.53 arcsec |
| 4 | 3072 | 3.665 deg | 25.77 arcsec |
| 5 | 12288 | 1.832 deg | 12.88 arcsec |
| 6 | 49152 | 54.97 arcmin | 6.442 arcsec |
| 7 | 196608 | 27.48 arcmin | 3.221 arcsec |
| 8 | 786432 | 13.74 arcmin | 1.61 arcsec |
| 9 | 3145728 | 6.871 arcmin | 805.2mas |
| 10 | 12582912 | 3.435 arcmin | 402.6mas |
| 11 | 50331648 | 1.718 arcmin | 201.3mas |
| 12 | 201326592 | 51.53 arcsec | 100.6mas |
| 13 | 805306368 | 25.77 arcsec | 50.32mas |
+------------+-----------------+--------------+------------------+
Definition at line 31 of file _hpxUtils.py.