Update analysis plan to preselected subjects based on fMRI scan quality

Subjects in the appendix updated. Training set numbers went from 390 to 362 and hold-out set numbers went from 98 to 91.

The following description was added to the dataset selection section:

"Before considering any depression-related data, we selected a subset of the main UKB data based on fMRI scan availability and quality. More specifically, our subset consists of subjects who have resting-state fMRI component amplitudes (UKB field 25754) and faces-shapes contrasts (UKB field 25765). The availability of these two fields for a given subject implies the completion of UK Biobank’s resting-state and task fMRI preprocessing pipelines, ensuring that all downstream subject selection includes subjects with high quality brain scans."