added beat tracking notebook

Table_of_Contents.ipynb

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  "metadata": {
   "name": "",
-  "signature": "sha256:bdbfa3ec9eb61c71cd974adaf3009224ed93c7ca79b24beed967e29b8ac8aba9"
+  "signature": "sha256:c6a059b92be11e7d9f5701a4d82552676e2f75709c9378340f65c1ee47d9de3a"
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@@ -58,6 +58,7 @@
       "1.  [Segmentation](notebooks/segmentation.ipynb)\n",
       "1.  [Signal Representations](notebooks/signal_representations.ipynb)\n",
       "1.  [Onset Detection](notebooks/onset_detection.ipynb)\n",
+      "1.  [Beat Tracking](notebooks/beat_tracking.ipynb)\n",
       "1.  [Exercise: Understanding Audio Features through Sonification](exercises/feature_sonification.ipynb)"
      ]
     },
@@ -142,7 +143,6 @@
      "source": [
       "1.  [Tonal Descriptors: Pitch and Chroma](notebooks/tonal.ipynb)\n",
       "1.  [Feature Extraction](notebooks/feature_extraction.ipynb)\n",
-      "1.  [Beat Tracking](notebooks/beat_tracking.ipynb)\n",
       "1.  [Tempo Estimation](notebooks/tempo_estimation.ipynb)\n"
      ]
     }