Cleanup the DDLGlobalRegistry and ExprEval singletons.

- A ClhepEvaluator is now part of the DDLElementRegistry.
- A DDLElementRegistry is now percolated where needed.

@ianna, can you look into it? I ran with `25.0`, `scram b runtests` and with the geometry dumper you mentioned in the other thread and I could not find any differences.

There is also a couple of tests which are not actually built (sources not mentioned in the build files) which we probably should remove (or fix).