Remove GSL dependency

[hombit]

GSL provides a fast gradient slope method we use for *Fit features, but also causes a lot of problems:

• It is GPL-licensed (as well as the corresponding Rust crate), so it is causing this project to be GPL-licensed (see Switch license to MIT #181)
• Wrapper crate doesn't distribute sources and build routines, so it is the only dependency user must have installed to build this package with the full functionality
• It is significantly faster than Ceres, but it is also has significantly worse precision, see https://ui.adsabs.harvard.edu/abs/2024ApJ...974..169D/abstract and example code in this repo.