Rename `Derivatives` and export

[langfzac]

Currently, the Derivatives structure is only used in intermediate derivative computations -- not to hold the final derivatives. It is also not exported in the module. A user wanting do parameter inference with NbodyGradient will want to preallocate this type and update them in-place.

• [ ] Rename Derivatives to better reflect usage
• [ ] Export new type
• [ ] Update Integrator methods to take new type as an argument to avoid allocating