John M. Bouman has been a constant for the Sargent Shriver National Center on Poverty Law.A legal aid lawyer since 1975, Bouman helped form the organization in 1996 through a merger between the Legal Assistance Foundation and the National Clearinghouse for Legal Services.He became president in 2007 and has overseen the group’s expansion as both an Illinois policy leader and a nationally recognized advocacy group for the poor.But that will change next year. Bouman and the Shriver Center announced last week the search …