Add FunctionNode and basic cosmology

[jeremykubica]

This is a relatively substantial PR that changes how the TDAstro DAG works in a few import ways:

1. ParameterizedNodes have a clearer list of what type of setters they accept and cleaner logic to determine which type to use.
2. Create a new FunctionNode class to the evaluation DAG that allows users to wrap a function, such as random number generation or cosmology computation. The FunctionNode is based off the ParameterizedNode so that it keeps sampling in sync when evaluating the function.
3. Add a redshift computation function to both support cosmology and demonstrate how the new function nodes work.

[hombit]

Looks good, than k you

[OliviaLynn]

Looks good

[mi-dai]

Looks good to me!