Adds a SchemaFactory to create schema classes from dicts, json files or json urls.

The declarative schema is meant to interface with the Schema repo (https://gitlab.memri.io/memri/schema), and to allow users to definite their own schemas in a similar manner.

Remaining issues:

• Auto-generate from_json method for each schema
• move all simple schema classes from PyMemri to schema repo?
• edge constraints from schema json ("sequenced" and "singular")
• Method to automatically perform PodClient.add_to_schema
• Needs simple interface for plugin devs

Remaining issues:

• from pymemri.data.schema import * will not import all schemas, since __all__ is autogenerated by nbdev.
• check difference between pymemri schema and central schema