in config files, using schemas loaded from plugins.

This allows validating networks and transforms in the config file even
including those loaded from plugins.

Implementing this exposed a few bugs:

* For some reason network schemas were never being validated properly;
  if they were it would have exposed the fact that each
  nets/net_name.yml file was not referencing 'nets/base.yml' correctly;
  the RefResolver should enter a URI "scope" for each schema such that
  references are resolved relative to the current schema.

  So for example if nets/spidna1.yml references nets/base.yml, this
  gets resolved to nets/nets/base.yml.  From within spidna1.yml the
  correct relative reference is just "base.yml".

* Also had to fix handling of resolving filename references while
  referencing a schema from a dnadna-plugin URI.

* The changes to training.yml look big, but actually most of it was
  just re-indented.  It went from:

      properties:
          model_name:
          ...

  to:

      allOf:
          - properties:
                model_name:
                ...
          - {"$ref": "dnanda-plugin:network"}

  where "dnadna-plugin:network" provides a schema that allows
  "network_name" only for those networks loaded from plugins, and
  associates the "net_params" property with the correct schema for
  the network selected by "network_name".