Fix writing of derivative file for clang build

in the clang build analyzer test, it looks like the size of the derivative file is smaller than it should be. This also causes the iterator test to fail

in gcc builds Equations::writeToFile: INFO:: wrote 480 events in 453820 bytes to file

in clang builds Equations::writeToFile: INFO:: wrote 480 events in 42552 bytes to file (this size seems to be independent of number of events)

It looks like part of the information is just not written to the file. My suspicion is that m_elements, which contains the main chunk of information, is not written to the file for clang builds.

https://gitlab.cern.ch/lhcb/Alignment/-/blob/master/Alignment/AlignKernel/src/AlEquations.cpp#L98