Implement `incrmwvariance`

Incremental moving weighted variance. `@stdlib/math/statistics/incr/mwvariance`.

Two use cases:

1. Fixed window weights. In which case, should provide a `factory` method to set the weights beforehand.
2. Weights which accompany each value: `accumulator( x, weight )`.

Start at @stdlib/math/statistics/incr/mwvariance and review the existing incremental statistics APIs around it. Define how the factory should accept fixed-window weights and how accumulator(x, weight) should handle per-value weights, then verify both use cases with focused tests and numerical examples.

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