Adjust notebooks to run on Colab, Kaggle, Gradient, and SageMaker

[fdabrandao]

I added links in the headers to load the Jupyter Notebooks directly on Google Colab and similar platforms and a cell to ensure the presence of the dependencies.

The changes are fairly minimal and should not affect the ability to run on binder:

• Links for the 1-RiskReturnModels.ipynb notebook (currently point to the version in master so the cell below is missing):

• The cell added to all notebooks that ensures that dependencies are there and clones the repository if necessary (some platforms load just the notebook by itself):

  !pip install pandas numpy matplotlib PyPortfolioOpt
  import os
  if not os.path.isdir('data'):
      os.system('git clone https://github.com/robertmartin8/PyPortfolioOpt.git')
      os.chdir('PyPortfolioOpt/cookbook')

You can see readonly versions of the notebooks running on Colab at:

1 - https://colab.research.google.com/drive/1VD2EpbIxJ4D7FKdIsAlhqtVgaPDZehT_?usp=sharing

2 - https://colab.research.google.com/drive/1uEBYdi8M2Vn0uRSrl6lYaQCds4ZqHVVq?usp=sharing

3 - https://colab.research.google.com/drive/1cSV3_h_irzBLeIovwmdetLudVejZzPuN?usp=sharing

4 - https://colab.research.google.com/drive/1EqiPqKju6u4zsMNgH5NTbQI3ybP8VClq?usp=sharing

5 - https://colab.research.google.com/drive/1HaM6HT6syD5iLkqaPjz5x2Ks8AMWIKFL?usp=sharing

[robertmartin8]

Looks good! Hoping to merge stuff in a couple of weeks' time. Sorry for the slow pickup!