SPECTUB seem to set first LOR in each view to empty

[KrisThielemans]

ProjMatrixByBin::get_proj_matrix_elems_for_one_bin checks if the LOR is in the cache, and if not it will call calculate_proj_matrix_elems_for_one_bin (and then store it in the cache, if caching is on).

ProjMatrixByBinSPECTUB::calculate_proj_matrix_elems_for_one_bin should fill in the LOR passes as argument. However, it doesn't:

STIR/src/recon_buildblock/ProjMatrixByBinSPECTUB.cxx

Lines 912 to 913 in 42aebde

	lor.erase();
	}

The same problem occurs for PinholeSPECTUB (as the code is (nearly?) identical).

@danieldeidda @mastergari do you agree?

Note that the all other LORs in the view will be cached, and hence will be fine.