Add explicit return type for lambdas

src/amdis/BoundaryManager.hpp

src/amdis/ProblemStat.inc.hpp

src/amdis/localoperators/ConvectionDiffusionOperator.hpp

src/amdis/typetree/Traversal.hpp

src/amdis/typetree/TreeData.hpp

src/amdis/utility/QuadratureFactory.hpp