Split into modules for data registry, management, and retrieval

[karimbahgat]

Split into several basic modules with very limited areas of responsibility:

• Data registry: Getting overview and returning basic info about registered datasets from yaml files. Other modules can also reuse its components to get dataset information.
• Data manager: Managing the available data for a registered data, by downloading and optionally building zarr archives for datasets. This might be something we want triggered in other ways, but providing API endpoints for now so that we can easily populate data from the FastAPI interface.
• Data accessor/retrieval: Accessing available data for a given dataset, with optional time and space filters. All other modules that access data should reuse components from here. For now includes an endpoint to retrieve filtered data and download as netcdf file. Serves same purpose and should be easy to reuse and expand for OGC EDR endpoints.