RdfStore instead of N3store, and changed types to RDFJS types

[pietercolpaert]

• Using RdfStore instead of N3.Store. This will be interesting to test different indexes for performance and memory footprint, as we mainly use object look-up.
• Switched as much as possible to RDFJS types, which is also required by RdfStore

Made this branch for https://github.com/pietercolpaert/DCAT-AP-Dumps-To-Feeds/issues/7

[pietercolpaert]

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