refactor is-compatible ports with constexpr

[quesnel]

We known the type of output/input for each ports. So we can provide a constant
expression is-compatible port for each model.

enum class value_port_type : i8 {
	continuous,
	discrete,
	both
};

class counter {
	input_port x[1];

	constexpr value_port_type x_type(int i) noexcept
	{
		return value_port_type::both;
	}

};

class abstract_cross{
	input_port x[4];
	output_port y[3];

	constexpr value_port_type x_type(int i) noexcept
	{
		return value_port_type::both;
	}

	constexpr value_port_type y_type(int i) noexcept
	{
		if (i == 0 || i == 1)
			return value_port_type::continuous;
		else
			return value_port_type::discrete;
	}
};