Fix GCC 13 warning about ambiguity of operator==() in C++20

In C++20, a==b can also be implicitly rewritten as b==a, assuming
commutativity. But if the operator==(const &) is not declared as const, and is
thus asymmetric, this results in an ambiguity because neither of a==b or b==a
is a better match according to the overload resolution rules.

mex/sources/libkorder/tl/permutation.hh

@@ -1,6 +1,6 @@
 /*
  * Copyright © 2004 Ondra Kamenik
- * Copyright © 2019 Dynare Team
+ * Copyright © 2019-2023 Dynare Team
  *
  * This file is part of Dynare.
  *
@@ -101,7 +101,7 @@ public:
   {
   }
   bool
-  operator==(const Permutation &p)
+  operator==(const Permutation &p) const
   {
     return permap == p.permap;
   }

mex/sources/libkorder/tl/ps_tensor.hh

@@ -1,6 +1,6 @@
 /*
  * Copyright © 2004 Ondra Kamenik
- * Copyright © 2019 Dynare Team
+ * Copyright © 2019-2023 Dynare Team
  *
  * This file is part of Dynare.
  *
@@ -116,7 +116,7 @@ public:
     per.apply(nvmax);
   }
   bool
-  operator==(const PerTensorDimens &td)
+  operator==(const PerTensorDimens &td) const
   {
     return TensorDimens::operator==(td) && per == td.per;
   }