Issue 52: RMSE & MAE

[sheetalsattiraju]

What does this PR do?

• This PR adds rmse() (Root Mean Square Error) and mae() (Mean Absolute Error) as metrics in kstats-core/src/commonMain/kotlin/org/oremif/kstats/descriptive/RegressionMetrics.kt.
• Each metric is implemented with three overloads (DoubleArray, Iterable, Sequence) and uses the project's summation helper (compensatedSum()).
• A corresponding test file is included in kstats-core/src/commonTest/kotlin/org/oremif/kstats/descriptive/RegressionMetricsTest.kt, mirroring the structure of CentralMomentTest.kt.

Why was this PR needed?

• In issue Implement RMSE and MAE error metrics #52, the maintainer asked for these metrics to be added to kstats-core.
• RMSE and MAE are standard tools for evaluating prediction quality in regression, recommendation systems, and forecasting

What are the relevant issue numbers?

• Issue Implement RMSE and MAE error metrics #52

Does this PR meet the acceptance criteria?

• Tests added for new/changed behavior in kstats-core/src/commonTest/kotlin/org/oremif/kstats/descriptive/RegressionMetricsTest.kt covering known values, edge cases, invalid input, NaN propagation, and overload consistency (24 test cases total)
• All tests passing
• Follows project style guide — implementation mirrors the structure in existing CentralTendency.kt file
• No breaking changes introduced — 2 files added, no existing metrics or APIs were modified