Added some documentation in notebook for computing POP heat budget te…

…rms (#110)

* Adding a comment regarding the use of using grid.diff versus grid.derivative

* Adding a comment regarding the use of using grid.diff versus grid.derivative

* Adding a comment regarding the use of using grid.diff versus grid.derivative

* Adding comment about using grid.diff versus grid.derivative

docs/source/examples/CloseHeatBudget_POP2.ipynb

@@ -746,7 +746,8 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "#### i) Total heat advection"
+    "#### i) Total heat advection\n",
+    "<p style='text-align: justify;'> We use grid.diff and multiply and divide by the volumes ourselves is the way POP outputs the fluxes. It performs a division by the cell area before saving the terms, which would not be accounted for if we used grid.derivative. Note that we also multiply by dsxgcm.VOL.values and then divide by dsxgcm.VOL. This is due to the same issue, there is a mis-alignment in the grid in this output term that xgcm would not like, and we are getting around it this way. This might be specific to POP and it should likely be possible to use grid.derivative for other models.</p>"
    ]
   },
   {
@@ -964,7 +965,7 @@
    "name": "python",
    "nbconvert_exporter": "python",
    "pygments_lexer": "ipython3",
-   "version": "3.8.8"
+   "version": "3.9.1"
   },
   "widgets": {
    "application/vnd.jupyter.widget-state+json": {