Feature request: Improved path-finding and filling algorithms

[carshadi]

Explore alternate search algorithms and data structures so that users may tailor these parameters to better suit their individual setups. Currently, the searches are all backed by image-like arrays, which must be initialized with certain dimensions. For larger images, this can fill up a desktop or laptop's memory fairly quickly, so individuals with less RAM might benefit from more light-weight sparse matrix-like constructs.

This also ties into #50

Work on this is ongoing in the https://github.com/morphonets/SNT/tree/search-refactor branch