plot_sg() function requires sg_crit123 to be true (1) for all patients to show start and end of treatment

[milanwiedemann]

when using the plot_sg() function to visualise changes of scores around a sudden gain for two groups both groups need to have:

sg_crit123 = 1

when using the function to plot changes for one group that did have a sudden gain and another group that didnt have a sudden gain, this currently needs to be changed manually but overwriting sg_crit123 = 1 for all participants. the information about which group participants belong to (e.g., "group_variable") need to be stored in another variable and used in the group_var_name argument.