Add feature flags for numpy 1 and 2

By default, the extension is compile with support for numpy 1 and 2 (with runtime checks to pick the right binary offset where needed). Features or fields that are specific to a version are hidden by default. Users can opt-out of numpy 1 + numpy 2 by disabling default features and selecting a version. The library panics if the runtime version does not match the compilation version if only one version is selected.

Cargo.toml

@@ -31,3 +31,8 @@ nalgebra = { version = "0.32", default-features = false, features = ["std"] }
 
 [package.metadata.docs.rs]
 all-features = true
+
+[features]
+default = ["numpy-1", "numpy-2"]
+numpy-1 = []
+numpy-2 = []