Add Per Class Analysis and Class Confusion Matrix

[wtiandong]

1. Add per class dAP analysis.
2. Add Confusion matrix according to class errors
3. fix crash when there are only detection results in labels and segmentation section is empty list "[]".

[djaym7]

Can the author approve this ?

[naik-amey]

+1

[zivnachum]

This is great! I used it in my code, thanks!
@dbolya any chance this pr can be approved? :)

[yiftachbeer]

Until this PR is merged here, I've created a new fork where it is merged: https://github.com/yiftachbeer/tide

However @wtiandong note that get_per_class_APs in ap.py is indented with spaces instead of tabs as in the rest of the file.