Import CSV should merge new data if user exists

[iamkroot]

Currently, the import CSV function fails if the user with the same PK already exists in the database. It should instead update the database according to the new data from CSV.

[ujjwal-raizada]

what should be done in this case:
record in db: student1 => {'id_num: '1', email: 'a'}

new students to add:
student2 => {'id_num: '1', email: 'b'}
student3 => {'id_num: '3', email: 'a'}

id_num and email both are primary fields

[iamkroot]

We have considered id_num to be the pk for student, so that will take priority over email. (email is just candidate key). So in this case, whatever the CSV shows should be entered into db.

[iamkroot]

@ujjwal-raizada please finish this