ACTION-269 - associated pull request

Dear all,

Pull request https://github.com/w3c/sdw/pull/572 implements the changes for
ACTION-269:

This action is related to resolution 1: Observable Property in sosa,
Property in ssn, old SSN Property equivalent with ssn:Property as of Option
6 (a derivation of Option 5 several people have already indicated their +1)
on wiki:
https://www.w3.org/2015/spatial/wiki/Property_and_Observable_Property

See my inline comments on the changes that were operated in the files:
https://github.com/w3c/sdw/pull/572/files

Kind regards,
Maxime