Allow suffix modifier for subject ID lists

[mh105]

As we discussed at the methods-garage meeting, it might be more intuitive and efficient to allow suffix modifiers in the subject ID list txt file to provide more granular controls such as:

• Only run step 1 or step 2 for certain subjects
• Force rerunning a step or all steps for certain subjects
• Additional modifiers that are relevant

This would allow this template to keep one single subject ID list txt file while supporting and extensible for future modifiers to control how the pipeline handles different subjects differently.