[2pt] Add HAND SRC datum values to hydrotable.csv

[RyanSpies-NOAA]

The current dev is exporting the HAND thalweg reference elevation values for each pixel catchment as well as the median thalweg elevation value for each hydroid segment. This is the elevation datum for each of the synthetic rating curves. Let's include this as an attribute in the hydrotable.csv to facilitate rating curve comparisons (#269).