Common interface for the generators

[jcabot]

We should have a very simple interface that all generators should implement. Maybe one method would be enough (receiving as input the model and a set of parameters) but the idea is to be able to call any generator (e.g. as part of a pipeline) without having to adapt the code to every generator (e.g. imagine that the generator for python is called "generate", the one for sql is called "generate_for_sql",... I want to avoid this)