Abstract implementation for different executors

[JimCircadian]

Descendant from #26

The batch executors need to be processed out of batch_config. With the addition of the naive repeat parameter it's definitely valid to generate the executors dynamically from the configurations.

[thomaszwagerman]

@JimCircadian can you clarify this issue, if it is still relevant?

[JimCircadian]

Potentially @thomaszwagerman. I think this is the removal / decoupling of slurm configuration options that are currently mandatory into a configuration for the executor that is only required when you select a certain one. We can chat a bit more on this, but the batch config should contain things like repeat but not things like partition (which is slurm specific). I think this is where I was going with it. Potentially worth a chat? Not entirely sure how to make this user-friendlier (?) but keen that users don't need to stick redundant information in batch_config if they're only using a dummy executor, for example.