Assumed Radionuclide information in interfile header (PET and SPECT)

[varzakis]

When reading SPECT data without radionuclide information it defaults to 99mTc.

[KrisThielemans]

This is likely due to

STIR/src/buildblock/RadionuclideDB.cxx

Lines 228 to 240 in e03fbd0

	RadionuclideDB::get_radionuclide(ImagingModality rmodality, const std::string& rname)
	{
	// handle default case
	if (rname.empty() || rname == "default")
	{
	if (rmodality.get_modality() == ImagingModality::PT)
	return get_radionuclide(rmodality, "^18^Fluorine");
	else if (rmodality.get_modality() == ImagingModality::NM)
	return get_radionuclide(rmodality, "^99m^Technetium");
	else
	{
	warning(std::string("RadioNuclideDB::get_radionuclide: unknown modality. Returning \"unknown\" radionuclide."));
	return Radionuclide();