ENH: generalise name of extractor, extractors_used and similar variables

[jsheunis]

A more applicable naming would be metadata_source, since the concept of an extractor is not necessarily the only way in which metadata can be supplied to the catalog. A more general way of thinking about this is that a single metadata item can originate from some source, which in turn can be an extractor or aggregator or manual entry point or whatever else is a logical source of metadata.

Additionally, should look into making metadata_source a single object per metadata_item instead of (currently) a list. This would make it achievable to link one item to one source, which would allow the prioritisation process for metadata merging/replacement to work.

[jsheunis]

Link to metadata merging/replacement issue: #233

[jsheunis]

Closed by #237