Rename space out submobjects

[GoThrones]

Overview: What does this pull request change?

Name of the method was unnecessarily long. I've shortened it to space_out. It's intuitive to remember that a person wants to space out the constituents of a mobject.

Edit: The method "space_out_submobjects" is not used anywhere else in the source code.

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

[behackl]

No strong opinion on this, would like to hear what other maintainers think. The length of a method name usually isn't a great reason for introducing a breaking change.

[GoThrones]

Actually, iirc, space_out_submobjects is not used by any other method/function in mainmce. Does changing it's name still constitute a breaking change ?