Remove all mutables in TRT_ToT_dEdx
Mutables such as m_L, the length of a straw, are replaced by an output argument of a function. Since this calculation is done anyways in the standard configuration of the tool, it does not cost extra CPU. For non-standard configurations, a new function strawLength(Trk::TrackParameters) has been made available. The mutable m_isData is taken from the python configuration instead of from EventInfo.