Keyword argument to NumberLine constructor for double sided tips

[josiest]

Overview: What does this pull request change?

• Adds an additional keyword argument to the NumberLine constructor called include_double_sided_tips
• If this new kwarg is given True, it will add a tip to both ends of the numberline (ignoring the value of include_tip)

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

While it's pretty easy to add tick marks to both ends of a numberline yourself, it feels odd that there's an option to automatically add a tip to one end but not to both. Plus this saves a few lines of code.

Links to added or changed documentation pages

Further Information and Comments

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