Ehtim

Updates from winter 2023-2024

-added polcal_new function with gradients to speed up D-term calibratoin
-fixed an issue with assigning the MJD when reading certain dates
-fixed deprecation issues in rex

fixed bug in unpack_bl
verified figure display now works properly in matplotlib version > 3.7

- added functionality for Stokes V imaging
- added self-cal functionality for separate R & L gains
- added a new tutorial jupyter notebook
- improved space VLBI data generation, replacing pyephem with skyfield
- fixed many docstrings
- fixed bugs in display for latest matplotlib vresion
- fixed bugs in caltable

- multifrequency synthesis helper functions
- added analytic gradients for self-calibration
- added butterworth filter for image smoothing

-updates to multifrequency synthesis
-better documentation for starwarps

-Added geometric modeling library
-Fixed bugs in fits & uvfits I/O