Update GSL requirement from ^6 to ^7

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

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

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

Cargo.toml

@@ -39,7 +39,7 @@ emcee = "^0.3.0"
 emcee_rand = { version = "^0.3.15", package = "rand" }
 enum_dispatch = "^0.3.9"
 fftw = { version = "^0.8", default-features = false }
-GSL = { version = "^6", default-features = false, optional = true }
+GSL = { version = "^7", default-features = false, optional = true }
 itertools = "^0.12"
 lazy_static = "^1.4"
 libm = "^0.2"