Generate vocabulary for works rdf_types

[elrayle]

Temporarily created a vocabulary in /lib/hydra/works/vocab/works_terms.rb

Need to generate this vocabulary once the terms and properties are decided. See samvera/hydra-pcdm#24 for information on how this was done for pcdm.

[jrgriffiniii]

I'm uncertain as to the priority of this, but https://github.com/samvera/hydra-works/blob/master/lib/hydra/works/vocab/works_terms.rb still does not follow the pattern referenced in PCDM here: https://github.com/samvera/hydra-pcdm/blob/master/lib/hydra/pcdm/vocab/pcdm_terms.rb