Convert polarpy to more generic routine that can be used by all future polarisation instruments

[sujaymate]

Updates to polarpy based on the discussions at the Nendaz workshop. The points discussed and agreed upon are:

1. The current polarpy repo will be converted to more generic repo which can fit polarisation data for any future instrument provided the data and response is given in the decided format.
2. It will be renamed to be a more generic name and pushed to pip so people can do pip install to do the fitting.

Action items to do:

• Write a generic coordinate transform function (@sujaymate) using the agreed upon convention.
• Split the current polarlike.py such that common routines and POLAR specific routines are separated.
• Decide a common (across instruments) input data format and edit the relevant routines to read this file.