Raster pessimist

[dnomadb]

This aims to replace https://github.com/mapbox/pxm/blob/master/pxm-prep#L62-L73 with a randomized, windowed approach. Essentially, instead of loading the entire array, and checking the entire band at once, it:

• (optionally randomly) loads windows one-by-one, and;
• exits as soon as it finds any valid pixels

cc @camillacaros @perrygeo

[dnomadb]

Current method (https://github.com/mapbox/pxm/blob/master/pxm-prep#L62-L73)

8.73s user 1.04s system 97% cpu 10.008 total

This method:

raster-tester isempty /private/tmp/rawsat.tif --randomize  0.43s user 0.10s system 96% cpu 0.547 total

[perrygeo]

:+1: