Fix deprecated lag_matrix argument names (filling, drop_missing)

[Hugo-W]

Summary

lag_matrix emits DeprecationWarning for two keyword arguments:

• filling (deprecated, replaced by another name)
• drop_missing (deprecated, replaced by another name)

Impact

32 DeprecationWarnings are produced across the test suite on every run. While not failures, they clutter output and the deprecated arguments will be removed in a future version, at which point callers will break.

Fix

Replace the deprecated argument names with their current equivalents in all call sites. Per the lag_matrix docstring:

• filling=... → use the current argument name
• drop_missing=... → use the current argument name

Affected files (call sites)

• tests/test_regression.py (multiple)
• tests/test_solvers.py
• tests/test_utils.py
• pyeeg/models.py (the fit method calls lag_matrix with both deprecated kwargs)

Reproduction

uv run pytest tests/ -q 2>&1 | grep DeprecationWarning

[Hugo-W]

Precise argument mapping

For callers updating their code, the deprecated kwargs map as follows:

Deprecated kwarg	Replacement
filling=0.	fill_value=0.
drop_missing=True	mode='valid'
drop_missing=False	mode='full'

So lag_matrix(x, lags, filling=0., drop_missing=False) becomes lag_matrix(x, lags, fill_value=0., mode='full').

The deprecated kwargs are accepted via **kwargs in lag_matrix (pyeeg/utils.py:188) and mapped internally at pyeeg/utils.py:259.