pac_model: permit lag on growth option

src/ComputingTasks.cc

src/ComputingTasks.hh

src/DynamicModel.cc

src/DynamicModel.hh

src/DynareBison.yy

src/ExprNode.cc

src/ExprNode.hh

src/ModFile.cc

src/ParsingDriver.cc

src/ParsingDriver.hh