Buiiding MOC from images located in ant FITS extension

[lmichel]

The current API allows to build a MOCs from a set of FITS files located in a given folder:

MOC.from_fits_images(path_l, max_norder)

It is however not possible to use the same convenient method when the images are in a FITS extension. This is a case e.g. when we want to build a MOC from exposure maps which are usually not in the primary extension.
In this case we have to build MOCs for each individual file by using MOC.from_fits_image(hdu, max_norder, mask=None) and to merge them. This is a bit boring.

Having a function like this:

MOC.from_fits_dhu(path_l, hdu, max_norder)

Would be very helpful.

[fxpineau]

The method now takes as optional parameter the index of the HDU iin FITS files,
see this line:

def from_fits_images(cls, path_l, max_norder, hdu_index=0):