migrate to coreforecast

[jmoralez]

Description

• Replaces most of the numba-based computations (target transformations, grouped array functions) with coreforecast.
• Adds the Combine and Offset lag transformations to easily extend the coreforecast-based lag transformations.
• Updates the documentation to prefer the built-in transformations vs the numba-based ones.

Checklist:

• This PR has a meaningful title and a clear description.
• The tests pass.
• All linting tasks pass.
• The notebooks are clean.

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