added some missing operators and a test for all operators

src/amdis/Operators.hpp

src/amdis/assembler/ConvectionDiffusionOperator.hpp

src/amdis/assembler/FirstOrderDivTestvec.hpp

src/amdis/assembler/FirstOrderGradTest.hpp

src/amdis/assembler/FirstOrderPartialTest.hpp

src/amdis/assembler/SecondOrderPartialTestPartialTrial.hpp

test/CMakeLists.txt

test/OperatorsTest.cpp