Update gnuplot requirement from 0.0.40 to 0.0.41

Updates the requirements on [gnuplot](https://github.com/SiegeLord/RustGnuplot) to permit the latest version.
- [Commits](https://github.com/SiegeLord/RustGnuplot/commits)

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

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

examples/observer/Cargo.toml

@@ -9,5 +9,5 @@ publish = false
 argmin = { version = "*", path = "../../argmin" }
 argmin-math = { version = "*", features = ["vec"], path = "../../argmin-math" }
 argmin_testfunctions = "*"
-gnuplot = "0.0.40"
+gnuplot = "0.0.41"
 instant = "0.1.12"