Test 019 segfaults sometimes

```
######## GMML TESTS COMPLETED ########
Required tests: 22
Passed tests: 21
Failed tests: 1
Time taken: 34 seconds
######################################

!!! OUTPUT OF THE 1 GMML TEST(S) THAT FAILED !!!

Testing 019.test.newPDBClass.cpp... ~30 seconds. ./019.test.newPDBClass.sh: line 14: 1455 Segmentation fault (core dumped) ./newPdbClass tests/inputs/"${file}" > 019.output.txt
Test FAILED!. 019.output.txt different from tests/correct_outputs/019.4mbz.pdb-output.txt
Compare using diff

Exit Code: 1

!!! FINISHED PRINTING FAILED TESTS !!!

webdev@oliver-gw-grpc-delegator:/programs/gems/gmml/tests$ bash 019.test.newPDBClass.sh
Testing 019.test.newPDBClass.cpp... ~30 seconds. Test passed.
Exit Code: 0
019.test.newPDBClass.sh: line 32: return: can only `return' from a function or sourced script
webdev@oliver-gw-grpc-delegator:/programs/gems/gmml/tests$
```
This happens on bare metal too, and so far only when running with other tests as a batch. Re-running the test by itself, or in a batch often has everything pass. Given the intermittent behavior, and that it only shows up when running as a batch of tests, I suspect the threading code for bond by distance has a fatal flaw.