Add Time Information to ContactLogger

The current ContactLogger saves the cell id, neighbours and contact length. Including the time step when the contacts are recorded would be useful to have information about the duration of each contact. e.g., fout << time << "\t" << cellid << "\t" << nb_cellid << "\t" << i->second << "\n";

Additionally, an option for file separation (per time_step or cell) would be also very helpful .