Modernize fitness concept definition

[morinim]

template<class F> concept Fitness =
  OrderedArithmeticType<F>
  && (std::ranges::sized_range<F> || (requires { requires F(-1) < F(0); }));

should be changed with:

template<class F> concept Fitness = OrderedArithmeticType<F> && requires { requires F{-1} < F{0}; };

This, in turn, requires constexpr constructor:

constexpr fitnd(std::initializer_list<double>);

C++20 supports this feature but a very recent version of the compiler is required.

The new definition is simpler and more powerful (extending ordering check to multi dimensional fitness types).