feat: Make SerDe optional

src/linalg/naive/dense_matrix.rs

@@ -1,11 +1,14 @@
 #![allow(clippy::ptr_arg)]
 use std::fmt;
 use std::fmt::Debug;
-use std::marker::PhantomData;
+#[cfg(feature = "serde")] use std::marker::PhantomData;
 use std::ops::Range;
 
+#[cfg(feature = "serde")]
 use serde::de::{Deserializer, MapAccess, SeqAccess, Visitor};
+#[cfg(feature = "serde")]
 use serde::ser::{SerializeStruct, Serializer};
+#[cfg(feature = "serde")]
 use serde::{Deserialize, Serialize};
 
 use crate::linalg::cholesky::CholeskyDecomposableMatrix;
@@ -349,6 +352,7 @@ impl<'a, T: RealNumber> Iterator for DenseMatrixIterator<'a, T> {
     }
 }
 
+#[cfg(feature = "serde")]
 impl<'de, T: RealNumber + fmt::Debug + Deserialize<'de>> Deserialize<'de> for DenseMatrix<T> {
     fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
     where
@@ -434,6 +438,7 @@ impl<'de, T: RealNumber + fmt::Debug + Deserialize<'de>> Deserialize<'de> for De
     }
 }
 
+#[cfg(feature = "serde")]
 impl<T: RealNumber + fmt::Debug + Serialize> Serialize for DenseMatrix<T> {
     fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
     where