Design: Figure out a better interface for Relative Values

[OAGr]

Description of suggestion or shortcoming:

Instead of our current Relative Value import system, it would be nice to have some better interface for this.

Some ideas:

1. RelativeValues.make() or similar

interface = RelativeValue.makeInterface({name, description, data, clusters})
estimate  = RelativeValue.make(fn, interface?)

2. Use Tags

interface = RelativeValue.makeInterface({name, description, data, clusters})

@interface(interface)
fn(var1, var2) = ...

I imagine that structs would help here, or some kind of way of declaring function definitions.

[OAGr]

Structs could be really nice for this, maybe that should be prioritized soon.