Enable JAX backend

[timmens]

Caution

This PR is only open to showcase the required changes to enable JAX. When #804 is merged, we will start implementing the changes --that might be here or in a fresh PR.

Enable JAX backend

In this PR, we ensure that gettsim becomes fully operational with the jax backend and is tested accordingly such that future changes in the codebase that are not JAX-compatible trigger test failures.

Closes #515

Issues

To make GETTSIM fully JAX-operable, the tax and transfers function, returned by dags, needs to JAX-jittable. This requires all functions defined in GETTSIM and called internally to be JAX-jittable. In particular, they cannot use functions from numpy or array methods that only work on numpy arrays.

ToDo's

• Make sure that _vectorize_func() is actually used and at correct position in code
• Ensure that make_vectorizable can handle lambda functions

Testing

• JAX end-to-end tests (i.e., jax.jit the compute taxes and transfers functions)
• JAX unit tests (i.e., jax.jit individual functions and call them on JAX arrays)
• Run (some of them) JAX tests on GHA

Numpy replacement

• Replace numpy.function(...) by numpy_or_jax.function(...)
  • asarray
  • allclose
  • searchsorted
  • array
  • zeros
  • inf
  • full_like
  • min
  • max
• Write JAX compatible join
  Currently, join_numpy is implemented. Either (1) implement join_jax and call if required, or (2) rewrite join_numpy using Array API standard.

Datetime replacement/refactoring

• Make sure that numpy.datetime64 is used before running dags
  (We want to allow the user to pass in data frames with DateTime-objects. However, these should be transformed internally to day, month, and year ints.)

Piecewise Functions

• Rewrite piecewise functions code
  Either create (1) one version of the code for numpy and one of JAX, or (2) try to solve this problem using the Array API standard.

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 87.08%. Comparing base (33ef921) to head (6d19b9d).

Files with missing lines	Patch %	Lines
src/_gettsim/policy_environment_postprocessor.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #812      +/-   ##
==========================================
- Coverage   87.82%   87.08%   -0.75%     
==========================================
  Files          56       56              
  Lines        3976     3971       -5     
==========================================
- Hits         3492     3458      -34     
- Misses        484      513      +29     

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