Move specification of datagram and schema to YAML.

[PeterKraus]

The specifications for datagram and schema should be written in YAML, and validated using strictyaml, to keep consistent with dgpost and reduce the documentation effort. This will be an interface-breaking change, hence it's tagged for version 5.0.

The schema files themselves also should be YAML files.

[PeterKraus]

Dataschema is now implemented using Pydantic, and YAML inputs are supported. Datagram schema is coming in 5.0.