Get rid of singletons

[alxbilger]

I propose to avoid relying on some singletons. The first I have in mind is sofa::simulation::getSimulation and setSimulation. It prevents to run two simulations in the same program (or only in a limited manner).

[fredroy]

An other big singleton to refactor is ObjectFactory.
Especially bothering because its getInstance() is called in the RegisterObject function, which is called when the library is loaded.
One solution would be to move those RegisterObject() into the plugin initialisation (where it has been given a pointer to the ObjectFactory). The drawbacks are:

• cumbersome
• the init's plugin file would need all the headers of the components it need to register.

[fredroy]

Other significant singletons:

• message mechanism (dispatchers/handlers),
• idFactory from timers
• Factories (contact, intersectors)
• scheduling (TaskScheduler, registry)
• scene loaders