HW2: Close input files, cahnge writing format
HW2: Output the top 100 PMI scores into PMI_raw.txt file
Output the exactly 20 appearances with their PMI score into PMI_select.txt file

Signed-off-by: unknown <zkk12345@gmail.com>
HW2: change filw writing format
HW2: <BUG-FIX> sort the collocations with exactly 20 appearances inPmiAnalysis()