Update SpaceNet plot fns

[ashnair1]

Add option to plot prediction if present in sample dict

Plot works for the following cases:

• Plot only image
• Plot image and mask(label)
• Plot image, mask and prediction

Following cases won't work:

• Plot only prediction
• Plot only mask
• Plot mask and prediction
• Plot image and prediction

First two might not be that important. Not sure if we want to support the last two. If required this could be done in a follow up PR.

[isaaccorley]

In other datasets we generally only support image/target or image/target/predictions if predictions is available. I wouldn't worry about supporting the others.

[calebrob6]

Looks great, thanks!