Allow jO to be symbolic links

Changes introduced

DSID/mc.*.py can be a symbolic link

Tests

Without the change:

999xxx/999999/mc.MGPy8EG_N30NLOA14_EFTAWU_llll_NPeq1_chl1_1.py -> ../../500xxx/500010/mc.MGPy8EG_N30NLOA14_EFTAWU_llll_NPeq1_chl1_1.py
./scripts/check_added_files.sh                                      
Find files that have been added in last commit...
ERROR: file: 999xxx/999999/mc.MGPy8EG_N30NLOA14_EFTAWU_llll_NPeq1_chl1_1.py not in white list

After the change:

./scripts/check_added_files.sh
Find files that have been added in last commit...
OK: file: 999xxx/999999/mc.MGPy8EG_N30NLOA14_EFTAWU_llll_NPeq1_chl1_1.py in white list

Also a passed pipeline here: https://gitlab.cern.ch/atlas-physics/pmg/mcjoboptions/-/jobs/7512044

Needed in !265 (merged)

Issues resolved

Closes #87 (closed)