Show example feature importance plot

[kelly-sovacool]

Issues

• Resolves Add feature importance plot #210.

Change(s) made

• Creates an example feature importance plot in the parallel vignette.
• Slightly simplifies get_feature_importance() -- it doesn't need the train_data.

Checklist

(Strikethrough any points that are not applicable.)

• [ ] Write unit tests for any new functionality or bug fixes.
• Update docs if there are any API changes:
  • roxygen comments
  • vignettes
• Update NEWS.md if this includes any user-facing changes.
• The check workflow succeeds on your most recent commit. This is always required before the PR can be merged.