feat: resolving data references via FakeProcessor :gift:

[fkroeber]

Description

This PR introduces the FakeProcessor class which enables simulated executions of recipes without actually loading or processing any data. This is useful for two applications:

• Firstly, it enables a recipe to be resolved in terms of its data layer references. The referenced data layers are written to the cache during processing by the FakeProcessor and can be read from there accordingly.
• Secondly, it enables a complete decoupling of the cache_data parameter from preview_run in recipe.py. The data cache can now be initialised via a FakeProcessor run, which is significantly faster than the existing necessity to have a preview run. Accordingly, the default for cache_data can also be set to True. Note that the functionality of preview_run has been changed in that it now only executes the preview run which is no longer automatically followed by a main run with native resolution.

Type of change

Select one or more relevant options:

• [ ] Bug fix (change which fixes a bug reported in a specific issue)
• [x] New feature (change which adds a feature requested in a specific issue)
• [x] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Documentation update (improved documentation for existing functionality in the package)
• [ ] Repository update (change related to non-package files or structures in the GitHub repository)

Checklist:

• [x] I have read and understood the contributor guidelines of this project
• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation
• [x] I ran all demo notebooks locally and there where no errors

[fkroeber]

Something that remains to be included...

Current Status

With the introduction of the FakeProcessor the cache_data parameter is completely decoupled from the preview_run in recipe.py. However for the QueryProcessor in core.py, the preview run is still a pre-requisite for building a cache. The QueryProcessor can't utilise an existing initialised cache object when it's run in preview mode. This makes the previews unnecessarily slow.

Feature to be implemented

Since the cache initialisation doesn't require a preview run anymore but can be realised via FakeProcessor in a more efficient manner, preview run and cache should also be decoupled in core.py. Specifically, the handle_layer method needs to be adjusted by omitting the self._preview conditioning.

[fkroeber]

The latest modification tackles the decouping of preview_run and cache_data as discussed previously. The preview_run will now behave the same way a normal run in respect to the cache utilisation (i.e. if a cache from a previous FakeProcessor run is provided it will be used, otherwise the cache will be ignored).