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Given the following text description, write Python code to implement the functionality described below step by step
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Description:
 Generate a left cerebellum volume source space
Generate a volume source space of the left cerebellum and plot its vertices
relative to the left cortical surface source space and the FreeSurfer
segmentation file.
Step1:  Setup the source spaces
Step2:  Plot the positions of each source space
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Python Code:
# Author: Alan Leggitt <alan.leggitt@ucsf.edu>
#
# License: BSD-3-Clause
import os.path as op
import mne
from mne import setup_source_space, setup_volume_source_space
from mne.datasets import sample
print(__doc__)
data_path = sample.data_path()
subjects_dir = op.join(data_path, 'subjects')
subject = 'sample'
aseg_fname = op.join(subjects_dir, 'sample', 'mri', 'aseg.mgz')
Explanation: Generate a left cerebellum volume source space
Generate a volume source space of the left cerebellum and plot its vertices
relative to the left cortical surface source space and the FreeSurfer
segmentation file.
End of explanation
# setup a cortical surface source space and extract left hemisphere
surf = setup_source_space(subject, subjects_dir=subjects_dir, add_dist=False)
lh_surf = surf[0]
# setup a volume source space of the left cerebellum cortex
volume_label = 'Left-Cerebellum-Cortex'
sphere = (0, 0, 0, 0.12)
lh_cereb = setup_volume_source_space(
    subject, mri=aseg_fname, sphere=sphere, volume_label=volume_label,
    subjects_dir=subjects_dir, sphere_units='m')
# Combine the source spaces
src = surf + lh_cereb
Explanation: Setup the source spaces
End of explanation
fig = mne.viz.plot_alignment(subject=subject, subjects_dir=subjects_dir,
                             surfaces='white', coord_frame='mri',
                             src=src)
mne.viz.set_3d_view(fig, azimuth=180, elevation=90,
                    distance=0.30, focalpoint=(-0.03, -0.01, 0.03))
Explanation: Plot the positions of each source space
End of explanation
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