Download function in fathomnet-generate

[eor314]

An image download function for fathomnet-generate command line tool. Main changes:

• Function to download images from list of AImageDTO.
• Checks if image exists in specified directory.
• Parser argument to specify download directory as string, default to None.

This works fine with COCO, but I don't think it will intuitively plug in with VOC format.

[kevinsbarnard]

Thanks Eric! Bumped version to v0.4.2 and published.