feat: Adding Mode and Moments to summary statistics

[N3uralN3twork]

Directly addresses #104

Adds mode statistics and raw/standardized moment APIs to SummaryStatisticsExt, expanding the crate’s summary-statistics coverage.

• Adds tje mode(), modes(), and mode_axis() with first-occurrence tie handling.
• Adds scalar and vector APIs for raw and standardized moments.
• Updates documentation and adds tests for ties, axis behavior, empty inputs, and numerical results.