Update ndarray requirement from 0.15.0 to 0.16.1

Updates the requirements on [ndarray](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-0.15.0...0.16.1)

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

Signed-off-by: dependabot[bot] <[email protected]>

Cargo.toml

@@ -17,7 +17,7 @@ exclude = [
 ]
 
 [dependencies]
-ndarray = { version = "0.15.0", optional = true }
+ndarray = { version = "0.16.1", optional = true }
 
 [badges]
 travis-ci = { repository = "argmin-rs/finitediff", branch = "master" }