C++11: convert SymbolType to a class enum

src/CodeInterpreter.hh

src/ComputingTasks.cc

src/DataTree.cc

src/DynamicModel.cc

src/DynareBison.yy

src/ExprNode.cc

src/ModFile.cc

src/ModelTree.cc

src/NumericalInitialization.cc

src/ParsingDriver.cc

src/Shocks.cc

src/StaticModel.cc

src/SteadyStateModel.cc

src/SymbolTable.cc