Remove `Composite` Structure

[genematx]

It turned out that the features of the Composite structure introduced in #668 -- namely, the flat name space across all arrays and table columns in a container -- create more complexity then bring benefits. The same functionality can be implemented via specs on the container objects and enforced by the Python client.

New proposed solution:

1. A container, which a user wants to present with a flat namespace if tagged with a Composite spec.
2. Python client knows that it needs to list names of all table columns in addition to the arrays as the container contents, and requests like c[container_name/column_name] should work (even though, internally, the client would be using the container_name/table_name?column=column_name endpoint). Moreover, c[container_name/table_name] should also work as in a usual container.
3. Nesting is not allowed; e.g. a "Composite" container can only have tables and arrays as its children, not other containers.
4. Attempts to add an array or a table with conflicting (column) names results in client error (without even making any server requests).
5. Attempts to tag a container with already conflicting contents' names as a "Composite" also results in a error (on the client?).
6. If a client encounters a container tagged as "Composite" that has conflicting names in its namespace (or nested containers), it displays and treats it as a usual container ignoring the spec, possibly issuing a warning to the user (and an error if the user attempts to access table columns directly).

[danielballan]

Validation can be enforced server side. Tiled has configurable validators that verify any assumptions you like about a spec. It has access to the structure and structure_family, such that rules like "must not be a container" can be verified.

This code was developed for AIMM and only lightly used. It has never been used on the floor at NSLS-II for Bluesky data, but the intent was that it could be. It may need some polish. It may also need to be extended to work for this use case, as I'm not sure as written it can validate assertions about the child of a node.

tiled/tiled/server/router.py

Lines 2285 to 2305 in cb7c46f

	for spec in reversed(specs):
	if spec.name not in validation_registry:
	if settings.reject_undeclared_specs:
	raise HTTPException(
	status_code=HTTP_400_BAD_REQUEST,
	detail=f"Unrecognized spec: {spec.name}",
	)
	else:
	validator = validation_registry(spec.name)
	try:
	result = validator(metadata, structure_family, structure, spec)
	except ValidationError as e:
	raise HTTPException(
	status_code=HTTP_400_BAD_REQUEST,
	detail=f"failed validation for spec {spec.name}:\n{e}",
	)
	if result is not None:
	metadata_modified = True
	metadata = result
	return metadata_modified, metadata

[genematx]

Great, thank you! configurable validators sound like exactly what we need.

[danielballan]

I like this change because:

• All other structures in Tiled are very widely understood and widely implemented concepts. This composite structure family was an oddity.
• We've made similar changes in the past, demoting xarray_dataset from a structure family to a spec, a special case of container.

[danielballan]

Closed by #1093