The expression is now a path component rather than query param.

* dimension universe
* return butler config
* query datasets types now properly handles expressions

This significantly improves performance.

This means that the service runs at /butler

Use the same method we use when copying other mappings.
Deepcopy causes problems if we try to attach objects to a config
after reading the config (for example in client/server testing).

In some edge cases a field that is normally a string will
be replaced by an object that does not support BUTLER_ROOT.
Catch the type error.

This works around the problem of hardcoding the butler repo location
(which needs to be fixed in the future) for testing.

Testing needs to be expanded but this demonstrates that testing
can be done.

This is not going to be a production method but is there
as a demonstration. To make it work we had to implement
refresh in the server but the long term usefulness of this
needs thought. What happens if a dataset type or collection
is added to the server butler? How do the cached butler objects
in the server know they all need to update?

It's likely not necessary since this is a simple GET that
returns a dict, but playing it safe won't harm anything.

They will all fail anyhow if FastAPI is not installed.