Fix return types in RBF interpolation functions

[msimberg]

#871 made a partial fix, but the types still ended up slightly inconsistent. The generic _compute_rbf_interpolation_coeffs returns a dynamically sized list of ndarrays, as it depends on a dynamically sized list as input. The return type was incorrectly specified as a two-element tuple (it can be one or two elements long). #871 correctly made sure the cell/vertex-specific functions return a tuple, but with _compute_rbf_interpolation_coeffs returning a list, the return values should be explicitly converted in those functions instead. This PR changes all uses of lists to tuples instead.

[msimberg]

cscs-ci run default

[egparedes]

A couple of comments for discussion.

[github-actions]

Mandatory Tests

Please make sure you run these tests via comment before you merge!

• cscs-ci run default

Optional Tests

To run benchmarks you can use:

• cscs-ci run benchmark-bencher

To run tests and benchmarks with the DaCe backend you can use:

• cscs-ci run dace

To run test levels ignored by the default test suite (mostly simple datatest for static fields computations) you can use:

• cscs-ci run extra

For more detailed information please look at CI in the EXCLAIM universe.

[halungge]

I have nothing more to add.

[msimberg]

cscs-ci run default