[789-deprecate-content-of-globaldefinitions] LevelPair -> MeshIndex

FWI/src/FWIRandomMains.cpp

FWI/src/FWI_DGTS.cpp

FWI/src/Helper/ABCHandler.hpp

FWI/src/Helper/FWIHelpers.hpp

FWI/tests/FWITestLinearized.cpp

FWI/tests/FWITestLinearizedAdjoint.cpp

FWI/tests/FWITestLinearizedMatrix.h

FWI/tests/FWITestProcTransfer.hpp

FWI/tests/FWITestSTI.cpp