Remove ObserveModelMixin and refresh some files

[tephenavies]

What this PR does

Remove ObserveModelMixin and all references to it. Also updates dataset counter tool to v8.

Test me

http://ci.terria.io/remove-deps/#tools=1

Checklist

• [ ] There are unit tests to verify my changes are correct or unit tests aren't applicable (if so, write quick reason why unit tests don't exist)
• [ ] I've updated relevant documentation in doc/.
• [ ] I've updated CHANGES.md with what I changed.
• [x] I've provided instructions in the PR description on how to test this PR.