Resolves #293 

A property (publication_date) was defined in SSSOM, but was not added to
the SSSOM mapping and mapping set data model. Since a very similar
property (mapping_date) was already defined, this change is considered a
bugfix, rather than an added feature.

Resolves #304 

In this PR, we 

- clarify the scope of a number of SSSOM properties (e.g author,
reviewer and creator_id properties are clarified to be URIs, rather than
"strings")
- Drop the use of syntactic concerns such "pipe-seperated" which do not
belong in a general spec
- fix some minor typos

In response to a request on slack, adding more details to the docs on
how to curation mapping justifications that are manually curated.

Fixes #78 

- [x] `docs/` have been added/updated if necessary
- [x] `make test` has been run locally
- [x] tests have been added/updated (if applicable)
- [x]
[CHANGELOG.md](https://github.com/mapping-commons/sssom/blob/0.9.0/CHANGELOG.md)
has been updated.