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.
