Eigen decomposition required for bvec averaging

[tashrifbillah]

bvec averaging may not be correct, should be done based on eigen decomposition:

conversion/conversion/grad_avg.py

Lines 67 to 70 in 48f94ce

	print('Averaging gradients: ', ind_same)
	data_new.append(np.mean(data[...,ind_same], axis= 3))
	temp= np.mean(bvecs[ind_same], axis= 0)
	bvecs_new.append(temp/np.linalg.norm(temp))