Suggest signed integer index type

Author: Q-Minh

source/pbat/math/linalg/mini/Matrix.h

@@ -7,6 +7,7 @@
 #include "pbat/common/ConstexprFor.h"
 
 #include <array>
+#include <cstdint>
 #include <initializer_list>
 #include <string.h>
 #include <utility>
@@ -234,7 +235,7 @@ PBAT_HOST_DEVICE auto Unit(auto i)
     return Identity<TScalar, M, M>().Col(i);
 }
 
-template <int M, int N, class TScalar, class IndexType>
+template <int M, int N, class TScalar, class IndexType = std::int64_t>
 PBAT_HOST_DEVICE auto FromFlatBuffer(TScalar* buf, IndexType bi)
 {
     return SMatrixView<TScalar, M, N>(buf + M * N * bi);
@@ -254,7 +255,7 @@ PBAT_HOST_DEVICE auto FromFlatBuffer(TScalar* buf, TIndexMatrix const& inds)
     return A;
 }
 
-template <CMatrix TMatrix, class IndexType>
+template <CMatrix TMatrix, class IndexType = std::int64_t>
 PBAT_HOST_DEVICE void
 ToFlatBuffer(TMatrix const& A, typename TMatrix::ScalarType* buf, IndexType bi)
 {
@@ -289,7 +290,7 @@ ToFlatBuffer(TMatrix const& A, TIndexMatrix const& inds, typename TMatrix::Scala
     }
 }
 
-template <int M, int N, class TScalar, class IndexType>
+template <int M, int N, class TScalar, class IndexType = std::int64_t>
 PBAT_HOST_DEVICE auto FromBuffers(std::array<TScalar*, M> buf, IndexType bi)
 {
     using ScalarType = std::remove_const_t<TScalar>;
@@ -314,7 +315,7 @@ PBAT_HOST_DEVICE auto FromBuffers(std::array<TScalar*, K> buf, TIndexMatrix cons
     return A;
 }
 
-template <CMatrix TMatrix, int M, class IndexType>
+template <CMatrix TMatrix, int M, class IndexType = std::int64_t>
 PBAT_HOST_DEVICE void
 ToBuffers(TMatrix const& A, std::array<typename TMatrix::ScalarType*, M> buf, IndexType bi)
 {