feature: Add opt-in Kahan compensated summation

[rtmongold]

Closes #13

Note: Adds a public summation field on IterativeConfig (default Pairwise). Call sites that build the struct with a full literal need the new field; Default, from_parameters, and with_summation are unchanged in behavior for existing users.

Checklist

• CHANGELOG.md updated under ## [Unreleased] (required for behavior-facing changes)
• cargo test + cargo clippy --all-targets clean locally
• New public APIs have a doc example
• No unwrap/expect/panic on library paths (typed errors instead)

[kmolan]

I do not like changing function APIs to add a new "summation" arg that 99% of the users will never care about. That is why currently pairwise summation is default. Unfortunately Rust does not do default function args so that adds another complication.

I would instead add the summation arg in struct member definitions, with pairwise as default. Users must call builder pattern function to enable kahan sum if needed. So for example LinearApproximator::default().with_kahan_summation() to perform all operations with kahan summation instead of the default pairwise. Similar logic for quadratic approximator and also the iterative integrators. Leave the existing APIs unchanged, add logic in them internally to perform the correct summation using method that user chose during initialization.