0.4.3

What's Changed

• Move backmapping and solar_diff_rot from the SolarMACH object to the base package so that they can be used externally (fixes #62).
• Create a new backmapping_angle function so that this calculation is now centralized (and not spread as multiple copies over the whole code).
• Introduce astropy.units to backmapping, backmapping_angle, and solar_diff_rot
• Extend docstrings (#45)
• Fix pytest falling for PFSS figure comparison

Full Changelog: v0.4.2...v0.4.3