Polarization convention and IAU compliance

[d3v-null]

• Birli uses the same polarization convention as Cotter for backwards compatibility.
• The Aips117 memo is ambiguous about which polarization convention to use.
• This convention appears to currently be X as EW and Y as NS [Citation needed, it would be good to show some evidence here]
• The IAU convention appears to be the opposite, X as NS and Y as EW [Note: it would be good to point to where speicifically it says this]

This is contentious topic and there are arguments for both staying consistent, and switching to the "correct" convention. Birli should be able to accommodate both options.

The plan is to:

• introduce an optional command line flag that will flip from the polarisation convention
• give people some time to play with this flag and provide feedback on the implications of switching convention
• if the benefits outweigh the costs, then switch the default convention to IAU

[d3v-null]

suggestion from @JLBLine :

we should add 'IAUORDER' keyword to the uvfits header which is either F for False or T for True. People could have a switch in their pipelines that just assumes this is False if the header isn't present for backwards compatibility.

[JLBLine]

See this paper here with Figure 1 showing x = North and y = East as the IAU definition (the paper also cites the source of the definition). As for written evidence as to why most uvfits seem to have x = East, I can find an example in pyuvdata here that has x as being East (snippet of code that say the below - it's about beams, but should apply to uvfits as well I imagine):
# check the x_orientation
print(uvb.x_orientation)
east

[d3v-null]

pyuvdata calls this "pol_order", and the options are either "CASA" or "AIPS". I kinda prefer this way of naming things

https://github.com/RadioAstronomySoftwareGroup/pyuvdata/blob/main/pyuvdata/uvdata/ms.py#L1885

edit: oops, that's the ordering of the pols, not the labelling of X and Y.

https://github.com/RadioAstronomySoftwareGroup/pyuvdata/blob/11e7129af5b105f066e505abb3383c41af3d6bcf/pyuvdata/uvdata/uvdata.py#L4562

CASA ordering has cross-pols in between (e.g. XX,XY,YX,YY)
AIPS ordering has auto-pols followed by cross-pols (e.g. XX,YY,XY,YX)