Build symptom subform data frame
     
    
    build_sy_subform_merged_df(symptom_parentformtablenames, trial_data, pid)
 
     
    
    Arguments
- symptom_parentformtablenames
 
A vector containing parent table names for symptoms
 
- trial_data
 
A SecuTrial data object
 
- pid
 
column name of patient ID in data