quick inspection of image bounding box labels

[zzsi]

Several errors/warnings we want to be informed:

• If bbox coordinates are absolute, they should match image dimension.
• bbox should not be out of bound.
• Some categories may have too few labels.
• Some bboxes are too small as a percentage of the whole image.
• If two bboxes overlap with more than 50% IoU, it may be problematic.

We can implement this for pascal voc format first.

[mwharton3]

Include visualization too