[BUG] SPINE always returns zero for num_fragments attribute for Particles

[dkoh0207]

Describe the bug

SPINE always returns zero for num_fragments attribute of TruthParticle and RecoParticle

To Reproduce

import numpy as np
import sys, os

SOFTWARE_DIR = '/sdf/group/neutrino/koh0207/spine/' # Change this path to your software install

import yaml

# Set software directory
sys.path.append(SOFTWARE_DIR)

ANA_DIR = '/sdf/data/neutrino/koh0207/logs/spine/bnb_nu_corsika/default'
cfg = os.path.join(ANA_DIR, 'config.cfg')
# cfg = '/sdf/data/neutrino/koh0207/logs/spine/bnb_nu_corsika/shower_cut/config.cfg'
DATA_PATH = os.path.join(ANA_DIR, 'file_list.txt')
cfg = open(cfg, 'r').read().replace('DATA_PATH', DATA_PATH)

from spine.driver import Driver

cfg = yaml.safe_load(cfg)

# prepare function configures necessary "handlers"
driver = Driver(cfg)

data = driver.process(entry=0)

sum_truth_fragments = 0
for p in data['truth_particles']:
    sum_truth_fragments += p.num_fragments

reco_truth_fragments = 0
for p in data['reco_particles']:
    reco_truth_fragments += p.num_fragments

print(sum_truth_fragments)
Out [10]: 0
print(reco_truth_fragments)
Out [11]: 0

Expected behavior
All particles will have num_fragments = 0, which does not reflect reconstruction as we build particles from fragments.

Screenshots, histograms, event displays
N/A

Code base
Provide the following:

• SPINE: https://github.com/dkoh0207/spine/tree/dedx
• Singularity: /sdf/group/neutrino/images/develop.sif

Additional context
It would be nice to have num_fragments filled properly when we don't build Fragment objects.

Possible solution (optional)
N/A