Add image utils

[MLnick]

Many MAX models are based on image input. They all require basic utils such as:

• load image
• handle different formats (JPG, PNG etc)
• handle errors (trying to pass non-supported image, or non-image as input)
• possibly conversion (PNG -> RGB; color -> grayscale etc)

See e.g. https://github.com/IBM/MAX-Facial-Age-Estimator/issues/13 and IBM/MAX-Human-Pose-Estimator#12 (but this applies to most if not all MAX models that operate on image input).

[MLnick]

cc @djalova @bdwyer2 @feihugis @i-hong @ptitzler @zuhito

[MLnick]

Other ideas could be to include utilities for handling things such as bounding boxes and segmentation masks (e.g. converting to or between bounding box formats, polygon vs RLE encoding for masks etc).