#161 fix: Standardize deconvolution model outputs

[Sprengmeister-dev]

Standardizes deconvolution predict() output to match other models (samples as index, cell types as columns). Updates expected outputs and the deconvolution example accordingly. Fixes #161.

[marcbal77]

Thanks for the contribution @Sprengmeister-dev. The core logic looks correct and addresses the transposed output issue for deconvolution models. Before we can merge, please:

1. Run make format - Black formatting isn't passing. See development.md for details.
2. Run make test - Please confirm all tests pass locally before pushing.

The fix itself is clean and the approach is solid. Once formatting is addressed, this should be good to merge.

[sarudak]

LGTM