SSN - term-by-term integration is finished

Dear all,

Term-by-term integration is finalized, and a pull request has been issued:
https://github.com/w3c/sdw/pull/700/

Ontologies in the integrated folder have nearly the same content as the
ontologies in the 'rdf' and 'ssn_separated' folders, except for:
 - dc:source which I didn't kept in the integrated folder (but can be added)
 - terms are organized in modules and introduced in the same order as in
https://www.w3.org/2015/spatial/wiki/Terms

I have checked programmatically:
 - The syntax of the ontologies
 - There is no more mistake in sosa vs ssn prefixes (there were a few),
 - The ontologies are in OWL DL.

Last step will be to add some local restrictions to formalize the
"domainIncludes" and "rangeIncludes" annotations

We can delete rdf/sosa.ttl and ssn_separated/ssn.ttl now, as the documents
in the integrated folder are a lot easier to read (terms are introduced as
in https://www.w3.org/2015/spatial/wiki/Terms), more consistent in terms of
metadata (ssn only), have more ontology metadata, and less unused
annotation properties.

Best,
Maxime