Allow standardizing a DenseMatrix by shifting the data

[abelianbee]

Closes #414.

standardize keeps shift = -mean/std and mult = 1/std on the StandardizedMatrix, so sandwich expands the product into four terms, one of them outer(shift, shift) * sum(d). That is a centered second moment computed from uncentered sums, so when a column's mean is large relative to its standard deviation those terms are orders of magnitude larger than their sum and the significant digits cancel away. No regrouping of the algebra avoids it. Centering the data does.

This adds an opt-in materialize_shift flag to standardize. When set, the shift and multiplier are folded into a copy of the data and the returned matrix carries a zero shift, so the expansion never happens. The default is unchanged. _materialize_standardization returns None by default, so SparseMatrix and CategoricalMatrix ignore the flag and are never densified. StandardizedMatrix holds a reference to the pre-standardization matrix, so unstandardize() stays exact and free, which glum relies on in _glm.py.

Measured against a longdouble oracle, same matrix both ways, on an M1 Max:

n	p	dtype	rel err, expansion	rel err, materialized	speedup
5,000	1,000	float64	9.1e-09	3.2e-15	1.20x
1,000,000	100	float64	1.2e-07	4.3e-15	1.90x
200,000	50	float64	3.9e-02	4.4e-13	2.36x
5,000	1,000	float32	6.9e+00	2.1e-06	1.11x

Better on both axes, which is worth saying explicitly because the obvious objection is that you pay for a copy. You don't: the expansion does three p x p outer products and an extra transpose_matvec, where the materialized path is one BLAS-3 call on contiguous data.

Tests cover representation equivalence across all seven matrix fixtures with both flag values, operation equivalence for sandwich / matvec / transpose_matvec, and an accuracy regression against a longdouble reference. 5666 passed, 45 skipped, 4 xpassed.

I moved the unreleased changelog section from 4.2.2 to 4.3.0, since this adds functionality. Happy to put it back if you would rather keep the number.

Checklist

• Added a CHANGELOG.rst entry