Add example illustrating that adding quantities with different units it not possible

[vil02]

Add an example showing that it is not possible to add two quantities of different units.

The code like:

#include "../../CtUnits/CtUnits.hpp"
#include <iostream>
#include <type_traits>

int main()
{
    struct Meter
    {
    };
    struct Inch
    {
    };

    using value_type = double;
    using DistanceInMeters =
        ctu::Quantity<value_type, ctu::UdMap<ctu::UnitDimension<Meter, 1>>>;

    using DistanceInInches =
        ctu::Quantity<value_type, ctu::UdMap<ctu::UnitDimension<Inch, 1>>>;

    const auto dist_a = DistanceInInches(123.0);
    const auto dist_b = DistanceInMeters(321.0);
    std::cout << (dist_a + dist_b).get_value() << '\n';
    return 0;
}

will not compile, because it tries to add meters to inches.