describe custom target value and components in aggregate_time()

doc/source/tutorials/subannual_time_resolution.ipynb

@@ -90,6 +90,9 @@
    "source": [
     "The function also supports directly appending the aggregated data to the original `IamDataFrame`. You can also pass a a list of variables, or call [variables()](https://pyam-iamc.readthedocs.io/en/stable/api.html#pyam.IamDataFrame.variables) to perform the aggregation on all timeseries data.\n",
     "\n",
+    "A user can also manually set the \"target\" sub-annual value and the components to be aggregated;\n",
+    "for example, this can then be used to process an aggregate of hourly data to monthly values.\n",
+    "\n",
     "You will notice that the following cell returns a larger dataset compared to calling the same function above."
    ]
   },
@@ -99,7 +102,8 @@
    "metadata": {},
    "outputs": [],
    "source": [
-    "df.aggregate_time(df.variables(), append=True)"
+    "df.aggregate_time(df.variables(), value='year', components=['summer', 'winter'],\n",
+    "                  append=True)"
    ]
   },
   {