Updated coupled spring notebook for python 3

This is updated for python 3 since most notebooks usually run on Google Colab, which automatically runs on python 3

Author: mathemusician

ipython/CoupledSpringMassSystem.ipynb

@@ -162,7 +162,7 @@
       "with open('two_springs.dat', 'w') as f:\n",
       "    # Print & save the solution.\n",
       "    for t1, w1 in zip(t, wsol):\n",
-      "        print >> f, t1, w1[0], w1[1], w1[2], w1[3]"
+      "        print(t1, w1[0], w1[1], w1[2], w1[3], file=f)"
      ],
      "language": "python",
      "metadata": {},
@@ -183,15 +183,14 @@
       "# Plot the solution that was generated\n",
       "\n",
       "from numpy import loadtxt\n",
-      "from pylab import figure, plot, xlabel, grid, hold, legend, title, savefig\n",
+      "from pylab import figure, plot, xlabel, grid, legend, title, savefig\n",
       "from matplotlib.font_manager import FontProperties\n",
       "\n",
       "t, x1, xy, x2, y2 = loadtxt('two_springs.dat', unpack=True)\n",
       "\n",
       "figure(1, figsize=(6, 4.5))\n",
       "\n",
       "xlabel('t')\n",
-      "grid(True)\n",
       "hold(True)\n",
       "lw = 1\n",
       "\n",
@@ -220,4 +219,4 @@
    "metadata": {}
   }
  ]
-}
+}