Adds labels map argument when loading annotations

[rwood-97]

Summary

At the moment, when you load annotations, the AnnotationsLoader will auto-generate a labels map. This is useful for first round of annotations as you wont have one defined yet.
However, if you are doing a second round of annotations, you might want to use the same labels map as before to make sure the numbering is consistent. This option is implemented in this PR.

Describe your changes

This PR:

• Adds labels map argument when calling load() method of annotations loader.
• Forces sorting of the labels map
• Implements method for appending new values to labels map.

Checklist before assigning a reviewer (update as needed)

• Self-review code
• Ensure submission passes current tests
• Add tests
• Update relevant docs

Reviewer checklist

Please add anything you want reviewers to specifically focus/comment on.

• Everything looks ok?