AMSTERDAM (Reuters) - Signify , the world's largest maker of lights, on Friday reported higher fourth-quarter core earnings and said it expected to improve margins in 2020 despite "challenging market conditions."Adjusted earnings before interest, taxes and amortization (EBITA) was 232 million euros (195.53 million pounds) for the 12 months ended Dec. 31, up from 214 million euros in the same quarter of 2018. A company-compiled poll of analysts had put adjusted EBITA at 190 million euros. Sales..