Feature/refactor resource

[koenvo]

This changes the api of a Source.

The discover_datasets is replaced with find_datasets.

The find_datasets method should yield the available DatasetResources. Each DatasetResource contains one or more files, and a FileResource describes how to load the data. This can be:

1. Already contains the json data via json_content
2. Has a url defined with optional http_options
3. Custom file_loader which must accept a FileResource and optional File (current_file)

Example:

dataset_resource = DatasetResource(
    # Set the Identifier of the Dataset
    dict(
        competition_id=competition_id,
        season_id=season_id,
        match_id=match["match_id"]
    ),
    dataset_type=dataset_type,
    provider=self.provider,
    name=name,
    metadata=match,
    state=state
)

dataset_resource.add_file(
    last_modified=last_modified,
    data_feed_key="match",
    data_spec_version=match_data_spec_version,
    json_content=match
)