Use case ... is for discovery of data including polarization information

Actually this use case has been reinforced by scientific needs of two communities: radio astronomy and optical spectropolarimetry... So how can we take into account this use case in OBstap ? I see three possible solutions:

* existence of Polarization information of the data could be considered as a subproduct type. Something like Subproduct-type: polarization.Stokes could be added to productype "Cube" or spectrum... By the way the same could be true for "Multi band" imaging, which could be another SubProducttype... But what to do if subproducttype is not retained ?

* Actually the obs_ucd field tells us what is the measured quantity on the observable (or functionally dependant) axis. the existence of polarization could be described by a UCD second word "polarization.stokes" concatanated with a phot.* ucd.

* With both solutions it will be possible to tell that polarization information is there but it will not be possible to give the list of polarization states actually present in the data. Characterisation version 2 (see Char2 DM presentation in Nara for details) proposes a new dedicated axis: polarizationAxis with an enumeration attribute. This attribute can describe the list of polarization states actually present in the dataset. A new optional field with utype Char.polaAxis.enumeration could be added to the Query response to encompass this.