Remove cg.series and cg.pages

[alanorth]

I propose that we make the following changes to CG Core v2:

• Remove cg.series in favor of dcterms.isPartOf, as it is a Dublin Core field and the intended use is for a "related resource in which the resource is logically included." In fact it is already in CG Core v2, so we should really just remove cg.series and adjust the documentation.
• Rename cg.pages → dcterms.extent for page numbers, as it is a Dublin Core field and we have been using it for page numbers for years already

As I stated in #30, our preference on CGSpace is to use vanilla Dublin Core terms where possible, then CG Core v2 fields, and then custom CGSpace fields. Let me know what you think.

[marieALaporte]

@olatunike any objection? we definitively need a new minor version for all the changes