Greedy fitting and quadratic modes

Version 1.1.0 of KerrRingdown` adds two new features.

1. Greedy Fitting: Any number of QNMs in the fitting model can have their expansion coefficient specified. These modes are then not fit, but are effectively subtracted from the source modes.
2. Quadratic Modes: Any number of quadratic modes can be added to the fit model. The full angular behavior of quadratic modes is not easily determined and currently their contribution is to a specific spin-weighted spherical harmonic mode. Thus they do not full fit within the true multi-mode fitting model.

While the functionality for these two features is now available, these features are not yet described in the Mathematica-Style Documentation. This will be added in a future update.