Separate Accelerator Class and Model Creation

[JoschD]

Content

There are a lot of functions in the accelerator class by now, that are only used for model creation.
As we also have individual model creators per accelerator, we should actually move this functionality from the accelerator class into model creation, hopefully decreasing coherence, making the code more readable and enabling easier adding of new accelerators,

The model creators would then get the specific (!) accelerator class as init-parameter and attach it to themselves,
using its parameters to e.g. create madx code etc.

The disadvantage is, that accelerator class and model creator class will have to be synced upon changes, but I don't think that can be avoided. And I hope that they are fairly static anyway.

Not an urgent task, but I think it would improve the code a lot - especially as model creation is one of the most convoluted codes in the repo.

Open for suggestions.