Obstap Polarization data discovery

[François Bonnarel]

To be more precise:
     - if we want to use the mandatory fields only o_ucd can be used to 
specify that phot.flux.density is completed by phys.polarization or 
phys.polarization.*. This gives a polarization "falvor" to the flux and 
can be searched by
ADQL queries.
      - this is sufficient for data discovery, but only the pol_states 
optional field  can give us the full list of available polarization 
states in the data set.