This PR adds an improved differential rotation model that is much faster and much better conditioned. There is still the issue that the degree of a differentially-rotated map increases without bound as time goes on. This is almost certainly not what happens in reality: magnetic forces should keep small spots from getting stretched into thin strips by differential rotation. The usefulness of this model may therefore be limited.
This PR adds an improved differential rotation model that is much faster and much better conditioned. There is still the issue that the degree of a differentially-rotated map increases without bound as time goes on. This is almost certainly not what happens in reality: magnetic forces should keep small spots from getting stretched into thin strips by differential rotation. The usefulness of this model may therefore be limited.