Avoid referring to private variables from outside of their scope

There are a lot of places in the code (many of which are my bad) where private variables are referred to in another context: for example, the `select-node-menu.js` file refers to `this.empress._featureMetadataColumns`, `this.empress._tipMetadata`, `this.empress._biom`, etc. pretty frequently.

Ideally, these references should be replaced with calls to new "getter" functions which return copies of the private variables / needed data. That, or these variables should no longer be annotated in the code as `@private` (and they shouldn't have the leading underscore characters).

This isn't a major issue or anything, but I think having this addressed will make testing a bit easier.

Start by auditing the cross-context references mentioned in select-node-menu.js, including _featureMetadataColumns, _tipMetadata, and _biom. Determine whether the project’s intended direction is getter functions returning copies or removing the private annotations and leading underscores. Done means the out-of-scope references are consistently addressed and the related testing behavior is easier to support.

Written by the indexing model from the issue text.