Generalise world to context

[MattWindsor91]

In discussion with Ana today we identified three different items that serve as a form of 'context' in a sequence diagram:

• the 'world' of a system (meant in a very specific way: this is the thing that a RoboWorld document will describe);
• the robotic platform of a module;
• the set of components available to a controller (presumably as well as the module services).

All of these are different variations on the same idea of a 'context', so to me it feels like there should be a generalisation of WorldActor to become, say, a ContextActor with an enum of different meanings for the context.

[MattWindsor91]

Decided against doing this; the 'world' actor is now just a blanket form of context.