fix: geneset() and snp_dataset() are deprecated but emit no DeprecationWarning

[Tanisha127]

Problem

geneset() and snp_dataset() have docstrings saying they are deprecated,
but they never call warnings.warn(). Users get zero indication at runtime
that they should switch to the new methods.

Steps to Reproduce

import warnings
warnings.simplefilter("always")
ag3.geneset()      # no DeprecationWarning raised
ag3.snp_dataset()  # no DeprecationWarning raised

Expected Behaviour

Both methods should raise DeprecationWarning pointing users to the
replacement methods genome_features() and snp_calls().

Files Affected

• malariagen_data/anoph/genome_features.py — geneset()
• malariagen_data/anoph/snp_data.py — snp_dataset()

Fix

Added warnings.warn(..., DeprecationWarning, stacklevel=2) to both methods.