FreeSurfer longitudinal requires a long_id column

[MatthieuJoulot]

Hi dear Clinica devs!

I am working up to use t1-freesurfer-longitudinal. I am encountering a small issue (which I have solved on my side). When using t1-freesurfer-longitudinal with the -tsv option the tsp requires a long_id column. This information is not provided in the documentation, and providing a specific template, meaning a subject has more than one template is more advanced, and should probably not be the default behavior.

I guess there is 2 things to do:

1. Remove the long_id column from the mandatory columns from the -tsvoption and get a flag (I guess) to tell the program when it should be looking for a long_id column in the tsv.
2. Write it in the documentation.

Have a good day,
Matthieu