Update ndarray-rand requirement from 0.14 to 0.15

Updates the requirements on [ndarray-rand](https://github.com/rust-ndarray/ndarray) to permit the latest version.
- [Release notes](https://github.com/rust-ndarray/ndarray/releases)
- [Changelog](https://github.com/rust-ndarray/ndarray/blob/master/RELEASES.md)
- [Commits](rust-ndarray/ndarray@ndarray-rand-0.14.0...ndarray-rand-0.15.0)

---
updated-dependencies:
- dependency-name: ndarray-rand
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

core/Cargo.toml

@@ -135,7 +135,7 @@ version = "0.5"
 
 [dependencies.ndarray-rand]
 optional = true
-version = "0.14"
+version = "0.15"
 
 [dependencies.rand]
 default-features = false