Simplify way to obtain peaks from mrtrix3

[DelinteNicolas]

elikopy/elikopy/individual_subject_processing.py

Line 3193 in b25db4b

fod2fixel_cmd = "fod2fixel -force -nthreads " + str(core_count) + " -peak peaks.mif" + \

Instead of three instructions, try to implement 'sh2peaks' directly then np.abs() the output.

[DelinteNicolas]

Fixed in #34