Refactor GeneratorBase

[cmnrd]

GeneratorBase in its current form is not very modular and flexible in its use. It appears to be mostly written with the C target in mind and does not consider the requirements of other targets in its design. For instance, it creates the instance graph twice, which is only used by the C generator. Also many other methods in GeneratorBase only apply to specific features like modes or federation, that are not supported by all targets. These features should better be factored out in separate classes which can then be optionally used by the different targets.

#1508 makes a start at this by factoring out the AST transformation for after delays into a separate transformation class, but there is still a lot more work needed.

Currently, the AstTransfrmation interface expects a list of reactors to apply the transformation to. This should also be revisited as past of fixing this issue. Ideally, we would pass the model or the resource to the transformation.