Fix for safeGet, where instead of nodata, it would return the boundary grid...

Fix for safeGet, where instead of nodata, it would return the boundary grid cell. Prevents unwanted discontinuities in calculations of curvature.

When using ListonWind, using a value of ETA that exceeds a few grid cells, noticeable discontinuities occur in wind speed. See the attached example for interpolated wind speed on a digital elevation model. X and Y units are hectometers. ETA was set to 5000m, so within 5km of the boundary (at X and Y=50), discontinuties occur (left figure). Right figure is after the proposed fix.