Merge segments module

This merge request adds the "segments" module (and its tests) to the codebase. The module is in dfha/segments.py and the tests are tests/test_segments.py

Overview: The "segments" module implements the Segments class. This class records the location of stream segments in a watershed, and allows users to calculate values for the stream segments from various data rasters. The stream segment pixels are typically a very small subset of the pixels in the data rasters, and so it is inefficient to perform computations over the raster as a whole. The "Segments" class addresses this issue by storing the locations of the stream segment pixels, and only performing calculations at those locations.