Fix bug in Cellwise FocalStrategy traversal.

The traversal should begin at the minimum row of the analysis area,
not at row 0 of the input raster.  This can cause fatal errors when
the analysis area extent has a y > 0.
Add benchmarks for focal operations on tiled rasters.

This checkin adds benchmarks for some focal operations on tiled rasters,
as well as an additional benchmark using a larger neighborhood.
Merge pull request #296 from joshmarcus/topic/focal_bug

Resolve bug with focal ops on tiled rasters and add benchmarks.