C++ generator still needs to centre module

[KrisThielemans]

PETSIRD/cpp/helpers/petsird_generator.cpp

Lines 74 to 75 in fa2f043

	{ 0.0, 1.0, 0.0, (rep1 - N1 / 2) * CRYSTAL_LENGTH[1] },
	{ 0.0, 0.0, 1.0, (rep2 - N2 / 2) * CRYSTAL_LENGTH[2] } } };

needs to use (Nx-1) as per the Python example.