Pillow Resizing Option for FFCV Dataset Creation

[warner-benjamin]

This PR adds the ability to use Pillow to resize images when creating a RGBField, which allows resizing images using antialiasing. It also adds an option to change the OpenCV interpolation, and a min_resolution setting.

I also changed the smart_threshold argstring to reflect what the code does. Perhaps the code the original argstring is referring too didn't get merged from a branch? Here is the current smart_threshold code:

if self.smart_threshold is not None:
    if image.nbytes > self.smart_threshold:
        write_mode = 'jpg'

[andrewilyas]

@warner-benjamin is this ready to be merged?

[warner-benjamin]

It's ready to go. I realized forgot to push my doc changes and fixed it yesterday.