Add more metadata to FITS headers

[zonca]

some files, even if in Equatorial, say reference frame is G in the FITS file.
Double-check for next release

[zonca]

Add all info I have about the map into the header (by Mathieu):

ORDERING, NSIDE, COORDSYS, UNIT, FREQUENCY, BEAMSIZE,
and it might also be useful, even if less crucial, to add to the header some COMMENT lines about the type of component and model, e.g. (CIB / Websky,  or Dust /PySM d0, etc)