writing of Interfile header fails for TOF data with 1 TOF bin

[KrisThielemans]

STIR/src/IO/interfile.cxx

Lines 1204 to 1205 in e168b07

	const bool is_TOF = pdfs.get_proj_data_info_sptr()->get_num_tof_poss() > 1;
	output_header << "number of dimensions := " + std::to_string(is_TOF ? 5 : 4) + "\n";

can be inconsistent with subsequent code if data is TOF anyway (e.g. if tof_mashing_factor == num_tof_bins, or if there's just 1 TOF bin), and the resulting header will be incorrect.

We should use ProjDataInfo::is_tof_data() instead.