Merge branch 'master' into paserticleset_add_variable_method

Author: erikvansebille

docs/examples/parcels_tutorial.ipynb

@@ -91,7 +91,7 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "The `fieldset` can then be visualized with e.g. `matplotlib.pyplot.pcolormesh()`. To show the zonal velocity (`U`), give the commands below. Note that we first have to load the fieldset with `fieldset.computeTimeChunk(time=0, dt=1)` to load the first time frame of the fieldset."
+    "The `fieldset` can then be visualized with e.g. `matplotlib.pyplot.pcolormesh()`. To show the zonal velocity (`U`), give the commands below. Note that we first have to load the fieldset with `fieldset.computeTimeChunk()` to load the first time frame of the fieldset."
    ]
   },
   {
@@ -111,7 +111,7 @@
     }
    ],
    "source": [
-    "fieldset.computeTimeChunk(time=0, dt=1)\n",
+    "fieldset.computeTimeChunk()\n",
     "\n",
     "plt.pcolormesh(fieldset.U.grid.lon, fieldset.U.grid.lat, fieldset.U.data[0, :, :])\n",
     "plt.xlabel(\"Zonal distance [m]\")\n",

parcels/fieldset.py

@@ -1141,7 +1141,7 @@ def write(self, filename):
                 if isinstance(v, Field) and (v.name != 'U') and (v.name != 'V'):
                     v.write(filename)
 
-    def computeTimeChunk(self, time, dt):
+    def computeTimeChunk(self, time=0., dt=1):
         """Load a chunk of three data time steps into the FieldSet.
         This is used when FieldSet uses data imported from netcdf,
         with default option deferred_load. The loaded time steps are at or immediatly before time
@@ -1150,9 +1150,12 @@ def computeTimeChunk(self, time, dt):
         Parameters
         ----------
         time :
-            Time around which the FieldSet chunks are to be loaded. Time is provided as a double, relatively to Fieldset.time_origin
+            Time around which the FieldSet chunks are to be loaded.
+            Time is provided as a double, relatively to Fieldset.time_origin.
+            Default is 0.
         dt :
-            time step of the integration scheme
+            time step of the integration scheme, needed to set the direction of time chunk loading.
+            Default is 1.
         """
         signdt = np.sign(dt)
         nextTime = np.infty if dt > 0 else -np.infty