Adding checks for nodata on the helper variables for the DETECT_GRASS...
Adding checks for nodata on the helper variables for the DETECT_GRASS algorithm, and providing a warning message when the DETECT_GRASS algorithm is ineffective due to missing data.
Also throw an error when DETECT_GRASS is TRUE, but ENFORCE_MEASURED_SNOW_HEIGHTS = FALSE.
Closes #896 (closed).
After MeteoIO issue is fixed, test again if there are no issues with nodata appearing in an operational setting.