Replace GeneratorFormat by something more scalable

[mgoeminne]

Currently, be.cetic.tsimulus.config.GeneratorFormat is used for mapping a JSON object to a time series generator. This object is basically a big map checking the value of the "type" attribute.

The idea is to provide something better, that would avoid to get this big list continuously growing as the number of generator increases.

An idea would be to place the responsibility to determine if a generator complies with the JSON object to the generator itself, which totally makes sense.