If you want to understand why Toshiba Corp. is about to report a multi-billion dollar write-down on its nuclear reactor business, the story begins and ends with a one-time pipe manufacturer with roots in the swamp country of Louisiana. The Shaw Group Inc., based in Baton Rouge, looms large in the complex tale of blown deadlines and budgets at four nuclear reactor projects in Georgia and South Carolina overseen by Westinghouse Electric Co., a Toshiba subsidiary.