NumPy Backend

[enekomartinmartinez]

I have been thinking a lot about numpy backend replacing the current xarray backend.

I think we also should rethink the subscript_dict between others to give more flexibility to the new backend but also to make the translated models more "editable". I will write down more details when I have time and the different options I thought about.

I suppose that this implementation will be slow, as currently, PySD is a side project for me. But this also gives me more time to develop and think better about the final implementation.