feat: Added ndarray_15 feature

src/ndarray_15_extra.rs

@@ -0,0 +1,15 @@
+pub trait Pow {
+    type Output;
+    fn powi(&self, rhs: i32) -> Self::Output;
+}
+impl<T, S, D> Pow for ndarray::ArrayBase<S, D>
+where
+    S: ndarray::Data<Elem = T>,
+    T: num::Float,
+    D: ndarray::Dimension,
+{
+    type Output = ndarray::Array<T, D>;
+    fn powi(&self, rhs: i32) -> Self::Output {
+        self.mapv(|x| x.powi(rhs))
+    }
+}