Add longitudinal processing

*   **What is the current behaviour?**  
Cross sectional processing for Free/Fastsurfer
*   **What is the motivation / use case for changing the behaviour?**  
Longitudinal processing of Free/Fastsurfer would enable more reliable results
*   **Other information**  
FastSurfer now has longitudinal processing https://deep-mi.org/FastSurfer/dev/overview/LONG.html that could be built into MicaPipe :) 