Update to parallactic angle calculation

[henry-ngo]

Hello,

I have been attempting some orbit fits using astrometry from VIP-processed NIRC2/vortex images as well as previously published astrometry.

I found that NIRC2_Preprocessing is computing the parallactic angle in a way inconsistent with most of the NIRC2 literature. In particular, there seems to be no correction for the orientation of the NIRC2 detector, as characterized by e.g. Service et al. 2016.

I have implemented an update on the alt_parang branch to add a new keyword that allows use of this distortion solution to compute the parallactic angle. You can see the diff here: master...alt_parang

I would like to determine the origin of the current implementation of the parallactic angle calculation. It would help us determine whether we should replace the calculation with the more commonly used Service+ 2016 solution or provide both choices (and which one should be default). Can someone who wrote the current get_parallactic_angles routine let me know how and why the current calculation is the way it is? @oabsil ? @CARLGOGO ? Thanks!

[oabsil]

This is a question for @owertz -- he was the one to implement this. I'm not sure how much testing / validation was done, though.