[nikosarcevic]

Closes #18.

• replace HaloProfileGeneric with a standalone DeltaSigma class that uses DSF's fftlog implementation without inheriting from CCL's HaloProfile.
• Update the data vec modeling code to use this new class
• small fix; i renamed the Hankel module to hankel_transform.py and updated the corresponding imports and documentation.
• added unit tests and a regression benchmark against the legacy ccl implementation, demonstrating agreement better than 0.05% across the tested radii and scale factors.
• Update the covariance integration and associated benchmark.

All tests and numerical benchmarks pass.