Better integration of halo exchange

[FlorianDeconinck]

The current version of the integration of halo exchange uses WrappedHaloExchange class to allow orchestration and stencil backends to co-exists (due to DaCe parsing limitations).

This has two issues:

• Goes against the "state-less" design of the code by requiring the same state to be passed to __init__ and __call__ of the model
• Performance of the orchestration is limited by the callback which acts as a black-box funnel for the data-flow

Solutions:

• Using literal list, which is pending a DaCe feature to be finished, we could keep the callback but remove the state from __init__
• Furthermore, we could recode the custom pack/unpack in a language that DaCe can parse then do away with the wrapper altogether