Bug: if all three volume dimensions are odd, result is NaN

[mathieuboudreau]

See: shimming-toolbox/b0-fieldmap-realistic-simulation#11 (comment)

This is because there will eventually be a division by zero here,

susceptibility-to-fieldmap-fft/functions/compute_fieldmap.py

Line 75 in 9a60de9

kernel = np.fft.fftshift(1/3 - kz**2/k2)

because k2 is simply,

susceptibility-to-fieldmap-fft/functions/compute_fieldmap.py

Line 72 in 9a60de9

k2 = kx**2 + ky**2 + kz**2

and for odd-numbered dimensions, kx, ky, and kz will contain zero in the middle element of their arrays due to this symmetrical linspace:

susceptibility-to-fieldmap-fft/functions/compute_fieldmap.py

Lines 66 to 68 in 9a60de9

	[kx, ky, kz] = np.meshgrid(np.linspace(-kmax[0], kmax[0], new_dimensions[0]),
	np.linspace(-kmax[1], kmax[1], new_dimensions[1]),
	np.linspace(-kmax[2], kmax[2], new_dimensions[2]), indexing='ij')