Warning needed for running two hippunfold processes at the same time to the same output folder

[mackenziesnyder]

To recreate the issue:

• run hippunfold-quick
• run another hippunfold process in a different terminal to the same output folder as hippunfold-quick, while hippunfold-quick is running

To fix:

• add a warning for the user when they attempt to run the second process that they need to switch output folders/wait for th first process to finish

@akhanf just to confirm these are the steps to recreate the issue?