Fix slice spacing computation in pydicom_series.py

[jond01]

The calculated distance_sum takes care only of ImagePositionPatient:

contrib-pydicom/input-output/pydicom_series.py

Lines 662 to 669 in cbd27f1

	# Finish calculating average distance
	# (Note that there are len(L)-1 distances)
	distance_mean = distance_sum / (len(L) - 1)
	# Store information that is specific for the serie
	self._shape = [len(L), ds2.Rows, ds2.Columns]
	self._sampling = [distance_mean, float(ds2.PixelSpacing[0]),
	float(ds2.PixelSpacing[1])]

However, to compute the slice spacing one also has to take care of ImageOrientationPatient, see dicom_numpy's implementation.