be explicit about parameters vs placeholders

[avibryant]

This just makes it clear where we're using Parameter vs Placeholder vs the generic Variable.

It also deletes some of the Viz stuff to be reintroduced later with support for placeholders and bounds.

This is setting things up for some larger changes around Placeholder.

[codecov-io]

Codecov Report

Merging #426 into 0.3-dev will increase coverage by 1.69%. The diff coverage is 70.37%.

@@             Coverage Diff             @@
##           0.3-dev     #426      +/-   ##
===========================================
+ Coverage    56.47%   58.16%   +1.69%     
===========================================
  Files           69       68       -1     
  Lines         2727     2644      -83     
  Branches       144      144              
===========================================
- Hits          1540     1538       -2     
+ Misses        1187     1106      -81

Impacted Files	Coverage Δ
.../scala/com/stripe/rainier/bench/SBCBenchmark.scala	0% <ø> (ø)	:arrow_up:
...com/stripe/rainier/bench/RegressionBenchmark.scala	0% <0%> (ø)	:arrow_up:
.../main/scala/com/stripe/rainier/core/Estimate.scala	0% <0%> (ø)	:arrow_up:
...c/main/scala/com/stripe/rainier/plot/Jupyter.scala	0% <0%> (ø)	:arrow_up:
...in/scala/com/stripe/rainier/core/Categorical.scala	44.44% <0%> (ø)	:arrow_up:
...in/scala/com/stripe/rainier/compute/Compiler.scala	92.59% <100%> (ø)	:arrow_up:
...in/scala/com/stripe/rainier/compute/Gradient.scala	93.22% <100%> (-0.23%)	:arrow_down:
...main/scala/com/stripe/rainier/core/Generator.scala	68.18% <100%> (ø)	:arrow_up:
...src/main/scala/com/stripe/rainier/core/Model.scala	75.86% <100%> (+11.15%)	:arrow_up:
...c/main/scala/com/stripe/rainier/compute/Real.scala	83.33% <100%> (+4.32%)	:arrow_up:
... and 4 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update 0bf50bd...b6a15a5. Read the comment docs.

[avibryant]

Nothing here that I think needs review in detail; this is more just for your awareness. The next PR will be more interesting.