Change Eigen::aligned_allocator to not inherit from std::allocator

Reference issue

Fixes #2895 (closed)

What does this implement/fix?

Problem with allocate_at_least inherited from std::allocator being called.

Additional information

I also added equal/not-equal operators to make the tests pass. This is also suggested at https://learn.microsoft.com/en-us/cpp/standard-library/allocators?view=msvc-170