Make `rborel()` return a numeric vector instead of an `<epichains_summary>` object

[jamesmbaazam]

This PR closes #191.

• Please check if the PR fulfills these requirements

• [x] I have read the CONTRIBUTING guidelines

• [ ] A new item has been added to NEWS.md

• [x] Tests for the changes have been added (for bug fixes / features)

• [x] Docs have been added / updated (for bug fixes / features)

• [x] Checks have been run locally and pass

• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...) Bug fix.

• What is the current behavior? (You can also link to an open issue here)

rborel() returns an object of class <epichains_summary>

• What is the new behavior (if this is a feature change)?

rborel() now returns a numeric vector.

• Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)

Not applicable.

• Other information: