Fix compile time bug in TimeIntegrator

In the recent major change introducing section dependent transport
model parameters the ParamID type has been changed from std::pair to
std::tuple. One place in TimeIntegrator.cpp has been forgotten and,
thus, was not adapted to the new type. This caused compile time bugs
since there is no ParamID constructor taking two arguments. This is
now fixed by giving -1 as third argument. Fixes #3.