Identify flat pixels in DEM

[wkearn]

Resolves #8

src/identifyflats.c adds the function identifyflats, which labels pixels in a DEM whose elevation is equal to the minimum elevation of their neighbors (flats) or whose elevation is equal to that of a neighboring flat (sills).

src/CMakeLists.txt adds src/identifyflats.c the library sources

include/topotoolbox.h adds the prototype for identifyflats

test/fillsinks.cpp is renamed to test/random_dem.cpp because we use random DEMs to test a variety of TopoToolbox functions

test/random_dem.cpp now runs identifyflats after fillsinks to label flats and sills in the DEM. The following properties are tested:

1. No flat pixel should have a neighbor that is lower than it
2. Every sill pixel should have at least one neighbor lower than it
3. Every sill pixel should border a flat pixel

These tests are incorporated into the loop over neighboring pixels.

test/CMakeLists.txt renames the fillsinks test to the random_dem test.