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This MR fixes many MT warnings in MdtSegmentT0Fitter.

The most significant ones are related to TMinuit. After discussing with @nkoehler and @rosati, it was decided to serialize the TMinuit fitter because it uses global variables to pass data to/from the fit function and there doesn't seem to be an alternative to that. The class will need to be reworked for unrelated reasons anyway.

The only remaining warnings are related to the initializer/finalizer use of the global extern gMinuit variable in the TMinuit header: https://github.com/root-project/root/blob/master/math/minuit/inc/TMinuit.h#L271