Restored virtual nature of MeshPrimitive/NurbsPrimitive setTopology().

Also added todos to remove it in the next major version. We still want to remove the virtual - we just feel that at this stage in the Cortex 8 development, it is time to lock down on compatibility in preparation for an 8.0.0 release.
Merge pull request #117 from johnhaddon/virtualTopologyMethods

Restored virtual nature of MeshPrimitive/NurbsPrimitive setTopology().
Bumped version to 8.0.0-a18 and updated Changes file.

Used GitHub markdown in Changes file so that it can be directly transferred to the release notes page on GitHub.
Removing invalid assert() commands.