Update GSL requirement from 2.0.1 to 4.0.0

Updates the requirements on [GSL](https://github.com/GuillaumeGomez/rust-GSL) to permit the latest version.
- [Release notes](https://github.com/GuillaumeGomez/rust-GSL/releases)
- [Commits](https://github.com/GuillaumeGomez/rust-GSL/commits)

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

Cargo.toml

@@ -14,5 +14,5 @@ keywords = ["simulation", "tree_sequences", "tskit", "population_genetics"]
 [dependencies]
 bitflags = "1.2.1"
 thiserror = "1.0"
-GSL = "2.0.1"
+GSL = "4.0.0"
 tskit = "0.1.1"