Track bounds for Real

[avibryant]

This tracks a (lower,upper) bound (as a double) for any Real, by starting each Variable at (-inf,inf), and starting each Constant(k) at (k,k), and then doing interval arithmetic to transform the bounds for any operations.

It works surprisingly well - eg Exponential(3).param.logistic.bounds gives Bounds(0.5,1.0).

The idea is then to have assertions in the constructors for distributions that enforce bounds like stddev has to be positive. This acts a bit like type checking for your models.

Before this is useful, we'll need to also add accurate bounds to Placeholder that are computed during Model.observe, otherwise we'll likely get assertion failures every time we use Fn. (We also need to add in the actual assertions).

[avibryant]

I'll add an issue for adding tests for this.

[codecov-io]

Codecov Report

Merging #420 into 0.3-dev will increase coverage by 1.07%. The diff coverage is 81.25%.

@@             Coverage Diff             @@
##           0.3-dev     #420      +/-   ##
===========================================
+ Coverage     54.9%   55.97%   +1.07%     
===========================================
  Files           69       70       +1     
  Lines         2683     2751      +68     
  Branches       149      150       +1     
===========================================
+ Hits          1473     1540      +67     
- Misses        1210     1211       +1

Impacted Files	Coverage Δ
...src/main/scala/com/stripe/rainier/core/Model.scala	64.7% <0%> (ø)	:arrow_up:
...c/main/scala/com/stripe/rainier/plot/Jupyter.scala	0% <0%> (ø)	:arrow_up:
...c/main/scala/com/stripe/rainier/compute/Real.scala	79.01% <94.44%> (+4.4%)	:arrow_up:
...main/scala/com/stripe/rainier/compute/Bounds.scala	96% <96%> (ø)
...ain/scala/com/stripe/rainier/compute/RealOps.scala	79.16% <0%> (+1.66%)	:arrow_up:

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