Pull request for action 301

Dear all,

Pull request https://github.com/w3c/sdw/pull/663 implements option 2 from
wiki page
https://www.w3.org/2015/spatial/wiki/Measurement_and_Operating_properties_for_actuators


Please note the following choices:

 - MeasurementRange is unchanged and only applies to sensors
 - ActuationRange is new, and only applies to actuators
 - Precision is unchanged (only applies to sensors)
 - Repeatability is new, and only applies to actuators
 - Sensitivity is unchanged (only applies to sensors)
 - all other sub classes of ssn:MeasurementProperty now apply both to
sensors and actuators.

As soon as we agree on this pull request and it is merged, I will isse
another one for the operating and survival ranges.

Best,
Maxime Lefrançois