Add pure python gt:debug backend

[FlorianDeconinck]

The current numpy backend as limitations:

• some operations of indexation cannot be conducted because they break broadcast rule
• readability is not great around mask and slicing boudaries
• the slicing, though faster, leads to a more complex NPIR and codegen
• debug-ability, on top of the issue of generated code, is not great

In order to create an area where quick iterative development of features can happen and start looking at debug tooling when writing model (which part of the tooling will need to address the nitty-gritty of the numerics) we propose to write a plain python "debug" backend.

This backend first and foremost shall be simplicity and closeness to C/Fortran looking code

Parent: GEOS-ESM/SMT-Nebulae#36

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• Write code-generation of a gt:debug backend straight from OIR or NPIR
• Document guiding principles, design decision and ideas/todos