Rearrange repository tree

[dr-shorthair]

The location of the artefacts in the repository is rather confusing at present. The directory containing the current rdf content is ssn/integrated, which is where @alexrobin has implemented his proposals, as described in https://github.com/sensiasoft/w3c_sdw/blob/oms-alignment/ssn/integrated/readme.md

Then there are a number of other directories containing supplemental artefacts:

• ssn/rdf an earlier version of the ontology, and some mappings which I think are still valid
• ssn/ssn_separated some artefacts related to the 2011 incubator draft and DUL alignment
• ssn-extensions the artefacts related to https://www.w3.org/TR/vocab-ssn-ext/
• ssn-usage inventory of known uses of SSN/SOSA as of 2017
• ssn-usecases ?

I propose to make it easier to navigate the repository as follows:

• put a brief README.md in each directory - it may contain little more than the above summary but at least there will be something
• rename rdf → mappings, move 20-082r2.ttl into a new directory oms, leave the mappings there, but remove everything else
• move ssnx and ssn-dul files to ssn_separated and rename it ssnx-dul

I hope that will preserve enough of the history but make things a bit more obvious.

Here is an image of the current directory tree.