Progressive's growth continues to slowBy Alistair Barr, CBS MarketWatch Last Update: 11:38 AM ET Jan. 20, 2005 SAN FRANCISCO (CBS.MW) -- Progressive Corp., the third-largest auto insurer in the United States, said Thursday that premium growth slowed in December compared with the previous month, continuing a trend that began last year. Progressive (PGR: news, chart, profile) , which reports results monthly, said net written premiums came in at $1.14 billion in December. Excluding an extra week that the insurer included in its December results, net written premium growth was 5 percent last month compared with December 2003, the company said....