Extract their names out of the generators

[mgoeminne]

Currently each generator may have a specific name. This is not very elegant since

1. Only top-level generator names are useful, but any generator may have a name.
2. All the generator objects have a Some[String] attribute for storing their name, and they don't really them.
3. By nature, the notion of "name" is not related to the notion of generator. The names are only used to refer to "external" generators in JSON documents storing such generators.

I propose to remove the "name" attribute from the generator objects, and to consider their name as a meta-information. From the perspective of JSON configuration documents, the "generators" attributes will therefore become:

"generators": {
   "name-here": {…}
}