Custom solution type

[fjebaker]

To facilitate a nicer analysis API, I suggest we move to wrap the SciMLBase types in our own struct, so that we can dispatch methods more targetedly. This would allow us to add plotting recipes unique to our package, allow overloads with the statistics community's fit functions, and generally let us be more expressive.

We could then also track which coordinates a given solution is in (Cartesian vs spherical), and provide mappings between them.

[fjebaker]

Closing, since we now basically do that with unpack_solution*.