used Storage trait for solve

see issue 667

src/linalg/qr.rs

@@ -170,7 +170,7 @@ where DefaultAllocator: Allocator<N, D, D> + Allocator<N, D>
         b: &Matrix<N, R2, C2, S2>,
     ) -> Option<MatrixMN<N, R2, C2>>
     where
-        S2: StorageMut<N, R2, C2>,
+        S2: Storage<N, R2, C2>,
         ShapeConstraint: SameNumberOfRows<R2, D>,
         DefaultAllocator: Allocator<N, R2, C2>,
     {