[L1-3] Rewrite note about *args; use len, not shape

lessons/01_phugoid/01_03_PhugoidFullModel.ipynb

@@ -343,7 +343,7 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "**Note**—We use the argument `*args` in the signature of the function `euler_step()` that is, then, passed to the function `f()`. `*args` is used to pass an arbitrary number of arguments to the function `f()`. Doing so, now, our function `euler_step()` can be used with any function `f()`, regardless the number of arguments this function takes. Sweet! (Read the Python documentation about [Arbitrary Argument Lists](https://docs.python.org/3/tutorial/controlflow.html#arbitrary-argument-lists) for more explanations.)"
+    "**Note**—We use an optional input to the function `euler_step()`, named `*args`. It passes to the function `f()` an arbitrary number of arguments. Doing so, `euler_step()` can take any function `f()`, regardless of the number of arguments this function needs. Sweet! (Read the Python documentation about [Arbitrary Argument Lists](https://docs.python.org/3/tutorial/controlflow.html#arbitrary-argument-lists) for more explanations.)"
    ]
   },
   {
@@ -546,8 +546,8 @@
     "        The difference between the two solutions in the L1-norm\n",
     "        scaled by the time-step size.\n",
     "    \"\"\"\n",
-    "    N_coarse = u_coarse.shape[0]\n",
-    "    N_fine = u_fine.shape[0]\n",
+    "    N_coarse = len(u_coarse)\n",
+    "    N_fine = len(u_fine)\n",
     "    ratio = math.ceil(N_fine / N_coarse)\n",
     "    diff = dt * numpy.sum(numpy.abs(u_coarse - u_fine[::ratio]))\n",
     "    return diff"