Clarification of initial polarisation for the POLARIMETER IDS

[imbeauf]

Copied from https://jira.iter.org/browse/IMAS-2968?filter=-1
It is a question initially raised by Rui Coelho, about
channel(i1)/polarisation_initial | Initial polarisation vector before entering the plasma {static} [m]

This node is ill defined.
Unless the codes try to calculate the Cotton-Mouton effect, knowing the initial polarisation is not useful.
If you have codes that already make use of the initial polarisation, then what is the definition they are using ? We could simply use a definition similar to what they expect (if it's generic enough).