issue-87: wiki page Property_and_Observable_Property to compare the proposals

Dear all,

Wiki page
https://www.w3.org/2015/spatial/wiki/Property_and_Observable_Property lists
different proposals for the integration of oldssn:Property in SOSA and the
new SSN.

Please add pros, cons, or other solutions.
Please modify the definitions if necessary.

This is related to ISSUE-87 and a solution to this issues shall be voted
during the next call.

Best,
Maxime