Prov-WD: Use Plates as example for Entity

[Ole Streicher]

Hi again,

section 2.2.1 "Entity and Collection classes" defines the entity and
lists some examples. It also describes non-digital entities (pg.13):

"Though the focus is on digital entities in this document, entities can
also refer to devices such as instruments or tools, that may be linked
to digital entities."

Since the current implementations more focus on the provenance of *data*
entities instead the provenance of tools or instruments, it would be
useful to bring a relevant example here. For physical entities, this
would be photo plates, which are entities in our APPLAUSE provenance
implementation. It is also unclear what "that may be linked to digital
entities" means.

Section 1 "Introduction" also has a (different) list here:

"In astronomy, such entities are generally datasets composed of
VOTables, FITS files, database tables or files containing values
(spectra, light curves), any value, logs, documents, or physical objects
such as devices or instruments."

Photo plates should be added here as well.

Both lists should also be harmonized; one can't write "devices or
instruments" in one place, and "devices such as instruments" on the
other place. IMO we should restrict this to one data and one non-data
example.

Best regards

Ole