Revert Vector2d changes to make it mutable

What does the merge request do?

!569 (merged)

closes #570 (closed)

• ImmutableVector2d:
  • ImmutableVector2d doesn't inherit Complex
  • Add complex method
  • Add eq for comparison between VectorLikes
• Vector2d:
  • Vector2d inherits from ImmutableVector2d
  • Add Vector2d's mutator methods back
• change the signatures of path function arguments to point : Union[complex, Vector2D]

Summary for release notes

This MR aims to revert the backwards incompatible changes made to inkex/Vector2d

Checklist

• Add unit tests (if applicable)
• Changes to inkex/ are well documented
• Clean merge request history