Mostly organizational improvements to downloading the original LOD files from Datahub.

* [DATAHUB_DOWNLOAD,DOC] Documented all predicates a little.
* [DATAHUB_DOWNLOAD,SC] Rearranged all predicates (main, helpers, messages, lexicographic order).
* [DATAHUB_DOWNLOAD,NEW] For easy of accorss-script retrieval of the same files based on a URL, we now store results in the MD-5 hash of the URL.
* [DATAHUB_DOWNLOAD,FIX] Convert the 'URL' to a URL. The 'URL' is sometimes an IRI.
* [DATAHUB_DOWNLOAD,NEW] Log the status of downloading and any messages thrown into a messages log file.
* [DATAHUB_GENERICS,NEW] New module housing predicates used by more than one module.
* [DATAHUB_LOD_PAIRS,SC] Shorter code due to (re)use of DATAHUB_GENERICS.