math: Fix C++17 std::iterator deprecation warning

GCC 15.2.1

In file included from image_io.cc:35:
../../libs/math/algo.h:145:38: warning: ‘template<class _Category, class _Tp, class _Distance, class _Pointer, class _Reference> struct std::iterator’ is deprecated [-Wdeprecated-declarations]
  145 | struct InterleavedIter : public std::iterator<std::input_iterator_tag, T>
      |                                      ^~~~~~~~
In file included from /usr/lib/gcc/x86_64-pc-linux-gnu/15/include/g++-v15/bits/stl_algobase.h:65,
                 from /usr/lib/gcc/x86_64-pc-linux-gnu/15/include/g++-v15/algorithm:62,
                 from image_io.cc:10:
/usr/lib/gcc/x86_64-pc-linux-gnu/15/include/g++-v15/bits/stl_iterator_base_types.h:129:34: note: declared here
  129 |     struct _GLIBCXX17_DEPRECATED iterator
      |                                  ^~~~~~~~

Author: andre-schulz

libs/math/algo.h

@@ -142,7 +142,7 @@ struct predicate_epsilon_equal
 
 /** Iterator that advances 'S' elements of type T. */
 template <typename T, int S>
-struct InterleavedIter : public std::iterator<std::input_iterator_tag, T>
+struct InterleavedIter
 {
     T const* pos;
     InterleavedIter (T const* pos) : pos(pos) {}