C++20 modernization: use contains() method on containers

src/ComputingTasks.cc

src/ConfigFile.cc

src/DataTree.cc

src/DynamicModel.cc

src/DynareMain.cc

src/EquationTags.cc

src/EquationTags.hh

src/ExprNode.cc

src/ExternalFunctionsTable.hh

src/ModFile.cc

src/ModelTree.cc

src/ParsingDriver.cc

src/StaticModel.cc

src/SubModel.cc

src/SubModel.hh

src/SymbolTable.cc

src/SymbolTable.hh

src/macro/Environment.cc