Figure 5: Update grdfilter to the new syntax (#37)

* Figure 5: Use readable arguments for the distance parameter

* Figure 5: Update grdfilter with filter/width

Author: seisman

Fig5_PyGMT_xarray.ipynb

@@ -12,7 +12,7 @@
     "\n",
     "mars = pygmt.datasets.load_mars_relief(resolution=\"10m\")\n",
     "with pygmt.config(PROJ_ELLIPSOID=\"mars\"):\n",
-    "    mars_filter = mars.gmt.filter(filter=\"g50\", distance=4)\n",
+    "    mars_filter = mars.gmt.filter(filter=\"gaussian\", width=50, distance=\"geo_spherical\")\n",
     "    mars_grad = mars_filter.gmt.gradient(azimuth=-45, normalize=\"cauchy\")\n",
     "\n",
     "fig = pygmt.Figure()\n",