TransformationMatrix: various "XformBy" methods were not needed

[pmolodo]

Description (this won't be part of the changelog)

ie, "translateBy" is redundant, because it calls the base implementation, which in turn calls "translateTo" with the updated value.

In addition to being cleaner, this update may provide a small performance boost, as we avoid calling pushToPrim twice every time an xformBy method is called

Also added a test to confirm behavior is still the same

Changelog

Changed

• removed unneeded TransformationMatrix::xformBy methods (ie, translateBy)
• added tests for proper functioning of xformTo/ xformBy methods

Checklist (Please do not remove this line)

• [X] Make sure the Title and Description of the PR make sense and provide sufficient context for your work
• [X] Do any added files have the correct AL Apache Licence Header?
• [X] Are there Doxygen comments in the headers?
• [X] Are any new features, behaviour changes documented in the .md format documentation?
• [X] Have you added, updated tests to cover new features and behaviour changes?
• [X] Have you filled out at least one changelog entry?