Dataset "GSHHG Earth distance to shoreline": Update docs and add coastlines to images (#126)

Co-authored-by: Dongdong Tian <[email protected]>

Author: yvonnefroehlich

docs/_static/GMT_earth_dist.jpg

@@ -51,7 +51,8 @@ We scale and reformat the original data to take up very little space so that dow
 from the servers are as fast as possible. For the distance grid this meant
 we chose 80 m as the smallest data unit, which is below the uncertainties in the
 model. Data are scaled and shifted to fit in a short integer grid that is highly compressed
-by netCDF lossless compression and chunking. The data are reported in kilometers.
+by netCDF lossless compression and chunking. The data are reported in kilometers. Positive
+(negative) distance values mean land to coastline (ocean to coastline).
 
 Data References
 ~~~~~~~~~~~~~~~

docs/_static/GMT_earth_dist_thumbnail.jpg

@@ -8,10 +8,13 @@ gmt set GMT_DATA_SERVER candidate
 let k=0
 I=""
 for set in ${list[@]}; do
-	gmt begin GMT_earth_${set} jpg
+    gmt begin GMT_earth_${set} jpg
         gmt grdimage @earth_${set}_10m -JQ0/7.5c -Rd ${I}
+        if [ set = dist ]; then
+          gmt coast -W1/0.2p,white -Df
+        fi
         gmt psconvert -A -P -E150 -FGMT_earth_${set}_thumbnail -Tj
-	gmt end
+    gmt end
     mv GMT_earth_${set}.jpg ../docs/_static/
     mv GMT_earth_${set}_thumbnail.jpg ../docs/_static/
     let k++