Fix promote_type with two complex floats and one double

[mnijhuis-tos]

While using xt::allclose on two objects containing std::complex<float>, I got an error from xtl::promote_type<std::complex<float>, std::complex<float>, double> that it tried determining the type when adding a std::complex<float> to a std::complex<double>, which is not defined.

When evaluating xtl::promote_type<std::complex<float>, std::complex<float>, double>, xtl indeed first recursively evaluates xtl::promote_type<std::complex<float>, double>, which yields a std::complex<double>. Then it determines the type of adding std::complex<float> and std::complex<double> using decltype and declval, which fails.

This fix avoids the addition operator in recursive calls of the promote_type template by generalizing the existing recursive template that already uses a bool as a first argument.

I also added a test that fails to compile without the fix and works fine with the fix.

I see that my editor removed some spaces and a final newline. Please let me know if that's a problem.

[JohanMabille]

Thanks for this fix!