FinRL_PaperTrading_Demo Numpy Array Transformation Failure

Running into issues with the `FinRL_PaperTrading_Demo.ipynb` tutorial, replicated both locally and on [the Google Colab](https://colab.research.google.com/github/AI4Finance-Foundation/FinRL/blob/master/tutorials/3-Practical/FinRL_PaperTrading_Demo.ipynb#scrollTo=Rzj0vjZZHdGM).

```
---------------------------------------------------------------------------

ValueError Traceback (most recent call last)

[](https://localhost:8080/#) in ()
----> 1 price_array, tech_array, turbulence_array = DP.df_to_array(data, if_vix='True')

2 frames

<__array_function__ internals> in hstack(*args, **kwargs)

[/usr/local/lib/python3.7/dist-packages/numpy/core/shape_base.py](https://localhost:8080/#) in hstack(tup)
343 return _nx.concatenate(arrs, 0)
344 else:
--> 345 return _nx.concatenate(arrs, 1)
346
347

<__array_function__ internals> in concatenate(*args, **kwargs)

ValueError: all the input array dimensions for the concatenation axis must match exactly, but along dimension 0, the array at index 0 has size 2190 and the array at index 1 has size 2186
```

The number of rows for each unique ticker don't match, hence the concatenation will fail. Is there some data cleaning step missing from this tutorial that needs to be added?