used Storage trait for solve method

as per issue 667 https://github.com/rustsim/nalgebra/issues/667

src/linalg/cholesky.rs

@@ -129,7 +129,7 @@ where DefaultAllocator: Allocator<N, D, D>
     /// `x` the unknown.
     pub fn solve<R2: Dim, C2: Dim, S2>(&self, b: &Matrix<N, R2, C2, S2>) -> MatrixMN<N, R2, C2>
     where
-        S2: StorageMut<N, R2, C2>,
+        S2: Storage<N, R2, C2>,
         DefaultAllocator: Allocator<N, R2, C2>,
         ShapeConstraint: SameNumberOfRows<R2, D>,
     {