Add gallery example for grdclip

Since noone worked on this issue during the scipy-sprint here's a gallery example adressing #1384.

examples/gallery/images/grdclip.py

@@ -0,0 +1,22 @@
+import pygmt
+
+fig = pygmt.Figure()
+
+# Load sample grid and use area around the Hawaiian Islands
+grid = pygmt.datasets.load_earth_relief(resolution="01m", region = [-162, -153, 18, 23])
+
+# Plot original grid 
+fig.basemap(region = [-162, -153, 18, 23], projection="M12c", frame=["f", '+t"original grid"'])
+fig.grdimage(grid=grid, cmap="oleron")
+
+fig.shift_origin(yshift = "-9c")
+
+# Set all grid points < 0 to a value of -2000.
+grid = pygmt.grdclip(grid, below = [0, -2000])
+
+# Plot clipped grid 
+fig.basemap(region = [-162, -153, 18, 23], projection="M12c", frame=["f", '+t"clipped grid"'])
+fig.grdimage(grid=grid)
+fig.colorbar(frame=["x+lElevation", "y+lm"])
+
+fig.show()