The SunPy coordinates framework

The SunPy coordinates framework is built off of the Astropy coordinates framework by extending it to handle coordinate frames relevant for solar physics.  `sunpy.coordinates` prioritizes accuracy in its coordinate transformations, and this accuracy has had multiple successes in uncovering errors in other libraries and data sources.  Complex examples of how `sunpy.coordinates` is used include:

• reprojecting an image taken of the Sun from one vantage point to a different vantage point
• accounting for solar differential rotation (i.e., the fact that different latitudinal bands of the Sun rotate at different rates)