Model separate volumes of periodicals

[osma]

Currently series membership (mainly from 830 fields) does not make use of volume number information. We should model the periodicals in more detail in the Schema.org output, probably using PublicationVolume.

See #45 where this surfaced

[osma]

See also:

• http://blog.schema.org/2014/09/schemaorg-support-for-bibliographic_2.html
• http://schema.org/PublicationVolume
• http://schema.org/PublicationIssue