Framework and methods for default initial conditions

[langfzac]

Implements two user-facing functions:

1. available_systems() which show the systems (and aliases) with implemented default initial conditions
2. get_default_ICs which returns an InitialConditions for the specified system

To implement more systems, one must implement a function that takes t0 and n (like get_trappist_ICs), and add keys to AVAILABLE_SYSTEMS and IC_FUNCTIONS.

[langfzac]

@ericagol Do you think we should add a few other systems before merging this?

[ericagol]

Yes, good idea! Kepler-36 would be good, but I have a lot of other favorites as well if you would like suggestions.

[langfzac]

This functionality is really useful, so for now I'm just going to merge and add other systems later.