Add Remove Dust Function

[william-silversmith]

This is probably one of the most common uses of cc3d and it's easy to screw it up and make it run slow.

Probably need to support three modes:

• remove fewer than this number of voxels
• remove smaller than this percent of the maximum size shape.
• remove smaller than this percent of the image size (can be done using option 1 and some math)

[william-silversmith]

# voxels? thresh > 1 but single voxels would be ambigious and have the opposite action
cc3d.dust(img, threshold=123) 
cc3d.dust(img, threshold=0.3) # fraction? 0 <= thresh <= 1

[william-silversmith]

All other functions can be achieved with computing image statistics and using the voxel dust function. This was completed in #84