feat: multi-source distance field

[william-silversmith]

Resolves #27

The following code:

import numpy as np
import dijkstra3d
from cloudvolume import view

field = np.ones((50,50), dtype=np.uint8)
sources = [ 
	(0,0),
	(49,40),
]


out = dijkstra3d.distance_field(field, sources)
view(out)

Produces:

Still need to add some automated tests. Euclidean distance field not included yet (?).