Catapulted by the market debut of his rocket company SpaceX, Elon Musk could become the world's first trillionaire by the end of the day.That level of wealth, all owned by just one person, was once unfathomable. Before Friday, the trillion dollar mark was reserved for measures like the GDP (or staggering debt ) of a handful of major economies - and, in the last decade alone, the value of some of the biggest companies to ever trade on the stock market.Musk's new title arrives amid a wider acceleration for the richest of the rich. Year after year, his former (although...