[FIX] AnimationPath will now also load/store its 'mode' property.

[lufriem]

Description

With this change the AnimationPath class will also read/write its 'mode' member; existing files will remain valid (their 'mode' member will not be changed).

Fixes #972

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

Using a modified vsgViewer example

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules