UCM13 — rename “executor” into “context” in the container model

The UCM container model specifies that a PortElementObject uses the name “executor” to refer to the ComponentObject it belongs to. The name “context” may be a better choice for this.
Indeed, the methods of a PortElementObject use the ComponentObject to retrieve the references to the other PortElementObjects of the component. So, the ComponentObject can be seen as a context for the execution of PortElementObject methods.
Using “context” would make UCM more consistent with CCM.

replace “executor” by “context”

In section 13, replace “executor” by “body” (1 replacement).
In the rest of the document, replace all occurrences of “executor” by “context” (4 replacements).
Update two diagrams that describe the UCM container model.