Uniform internal representation for metadata and datasets

Related #5 (closed)

We need a uniform representation for metadata to use for metadata returned from data connectors which implement DataConnectorHasMetadata.

A sensible representation will be dependent on what other APIs we expect to consume. The HyperCat representation of relation->value is good but may not be applicable to metadata from other sources.

IoTUK Nation does not provide metadata.

We also need a uniform representation for datasets - e.g. those contained within a HyperCat catalogue