(Reuters) -Private equity firm Warburg Pincus has struck a deal to buy Germany's PSI Software for more than 700 million euros ($811 million), with shareholder and customer E.ON remaining a strategic investor, the firms said on Monday. Reuters first reported on both a nearing takeover of PSI last week and Warburg Pincus' offer for the maker of software for energy networks on Sunday. Having jumped nearly 11% on Friday after the news broke, PSI shares gained a further 35% to trade at 44.9 euros apiece - the ...