Add keyword #linear-algebra for cargo.io discovery

[nielsle]

Consider adding the keywords #linear and #linear-algebra to cargo.toml. That would make the library more discoverable on https://crates.io. Here are crates.io pages for comparison.

https://crates.io/crates/ndarray-linalg https://crates.io/crates/rulinalg https://crates.io/crates/nalgebra https://crates.io/crates/lapack https://crates.io/crates/blas