Clang compiler produces the following warning which is treated as an error because it is located in MITK core. Fix it.

/run/media/cweber/Data/DartClients/mitk-clang-dartclient/MITK/Core/Code/Testing/mitkMatrixTypeConversionTest.cpp:58:40: error: adding 'unsigned int' to a string does not append to the string [-Werror,-Wstring-plus-int]

ss << "element [" << i << "][" + j << "] equal for mitkMatrix and podMatrix";
                          ~~~~~^~~

/run/media/cweber/Data/DartClients/mitk-clang-dartclient/MITK/Core/Code/Testing/mitkMatrixTypeConversionTest.cpp:58:40: note: use array indexing to silence this warning

ss << "element [" << i << "][" + j << "] equal for mitkMatrix and podMatrix";