Rename `<epichains_tree>` class to `<epichains>`

[jamesmbaazam]

This PR closes #185.

• 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, ...)

Feature.

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

The current S3 class for the object returned by simulate_chains() is called <epichains_tree>. This name was chosen to be descriptive of the tree structure of the object but has been deemed redundant.

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

The class is now renamed to <epichains>

• 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:

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 91.66667% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 98.78%. Comparing base (3c03275) to head (19d290f). Report is 5 commits behind head on main.

:exclamation: Current head 19d290f differs from pull request most recent head bd4407f. Consider uploading reports for the commit bd4407f to get more accurate results

Files	Patch %	Lines
R/epichains.R	90.90%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #218 +/- ## ======================================= Coverage 98.78% 98.78% ======================================= Files 8 8 Lines 574 574 ======================================= Hits 567 567 Misses 7 7 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.