Update basic_functions.py

completed identify_id_column, such that more than MIN_RATIO proportion of values in a column should have 8 digit values for it to be identified as student ID column
minor modifications
Merge branch 'patch-3' of https://github.com/gaurav1780/exam_processing into gaurav1780-patch-3
Merge branch 'gaurav1780-patch-3'
merged both pull requests