fact and perspective taking

[Greg8978]

For now, we use factObservability to tell if a fact can be perceived by others.
This should be improved by using observabilitySub and observabilityTarget.

Indeed, for some properties, it could make sense to update a fact when:
subject is perceived
target is perceived
both at the same time

as an example, if an agent A believes that an object O is on a furniture F:
O isOn F
If A sees F, he will know that A is no longer on F

having observability on subject and target will allow this.

1. First way to do this is to extend the fact definition

2. An other way to do this would be to have a configuration file that tells the observability policy according to the property.
   One limitation of this is that observability could also change according to the subject or target other properties.

As an example, if the fact is O1 isIn O2, the observability of this fact will depends on the transparency of O2, or if O2 is open or closed...

However, the pros is that it would make the fact lighter and give a more generic processing.