Fixed NumberLine get_number_mobjects() and get_labels()

[BillyLjm]

Overview: What does this pull request change?

Closes issue #2473, fixes NumberLine.get_number_mobjects() and the adjacent NumberLine.get_labels().

Motivation and Explanation: Why and how do your changes improve the library?

• NumberLine.get_number_mobjects() used an unimplemented function and would a throw an error if used. This simply fixes it to use the return the mobject of all ticks in the number line.
• NumberLine.get_labels() did not return the same thing as NumberLine.add_labels() which seems like a bug. So I fixed it to return self.labels, and default to the existing implementation if the attribute is not accessible.

Links to added or changed documentation pages

None

Further Information and Comments

None

Reviewer Checklist

• The PR title is descriptive enough for the changelog, and the PR is labeled correctly
• If applicable: newly added non-private functions and classes have a docstring including a short summary and a PARAMETERS section
• If applicable: newly added functions and classes are tested