Refactoring class to replace method call in class and define private

I have replaced method call in Class from self.method() to method().
However, I am still confused in both.

Also, I changed some public methods to private.
Save evaluation result into csv file

For convinience, two results of evaluation can be exported into csv
file.
Fixed #2
Test export evaluations into csv file

usage:
$ ruby ace_driver.rb demo/data/labels/labels.txt demo apk eval_apk.txt
$ ruby ace_driver.rb demo/data/labels/labels.txt demo case eval_case.txt
Refactor class methods to make them clear and efficient

I think calc_eval_per_case and calc_eval_per_apk might return same
type of result, not multiple array, but just a array. This is
because it makes both more clear to use.

Also, during this modification, I fixed a problem that eval_per_apk
stores not precise type of array. It should be 2D array same as
the type of eval_per_case. This problem made contents of exported
csv file weired.