Update nalgebra requirement from 0.23.0 to 0.31.0 (#128)

Updates the requirements on [nalgebra](https://github.com/dimforge/nalgebra) to permit the latest version.
- [Release notes](https://github.com/dimforge/nalgebra/releases)
- [Changelog](https://github.com/dimforge/nalgebra/blob/dev/CHANGELOG.md)
- [Commits](https://github.com/dimforge/nalgebra/compare/v0.23.0...v0.31.0)

---
updated-dependencies:
- dependency-name: nalgebra
  dependency-type: direct:production
...

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Cargo.toml

@@ -20,7 +20,7 @@ datasets = []
 
 [dependencies]
 ndarray = { version = "0.15", optional = true }
-nalgebra = { version = "0.26", optional = true }
+nalgebra = { version = "0.31", optional = true }
 num-traits = "0.2"
 num = "0.4"
 rand = "0.8"