Optimize PitRemove algorithm

[jcphill]

Modified PitRemove algorithm to only update neighbors of points that
have changed. Old algorithm would check every point that not had
all water removed on every pass, which was very slow for large input
DEMs with large pit areas. New algorithm uses a priority queue to
order updates by lowest filled elevation, which for a serial run will
update each point only once, and is better for parallel runs also.