Fix setting CesiumGeoreference.ellipsoid

[brendan-duncan]

Description

Setting CesiumGeoreference.ellipsoid currently does not work as intended at runtime.

In the editor, CesiumGeoreference.ellipsoidOverride is set, with a call to CesiumGeoreference.ReloadEllipsoid() to ensure the cached _ellipsoid is set up and all transforms updated. However, ellipsoidOverride is private and is not accessible at runtime.

This PR changes the setter for ellipsoid to instead set the _ellipsoidOverride and call ReloadEllipsoid(), providing the functionality that was previously only available from the editor.

Issue number or link

#636

Author checklist

• I have submitted a Contributor License Agreement (only needed once).
• I have done a full self-review of my code.
• I have updated CHANGES.md with a short summary of my change (for user-facing changes).
• I have added or updated unit tests to ensure consistent code coverage as necessary.
• I have updated the documentation as necessary.

Remaining Tasks

Testing plan

Tested the changes in the context of a Unity project. Changing Georeference.ellipsoid previously had effectively no effect. After the change the Georeference calculations properly used the new ellipsoid.

[j9liu]

Thanks @brendan-duncan, we'll take a look soon! Can we trouble you to update CHANGES.md for this fix when you get the chance?

[brendan-duncan]

Sure, no problem. Updated.