Portfolio docstring example fixes and some bugs

[montezdesousa]

Description

• Fixes docstring examples with errors
• Fixes metric sub commands units changing to absolute values, ratios were being displayed in % format
• Fixes metric volatility units
• Fixes bug where portfolio controller was not displaying correct risk free rate in print_help
• Changes PortfolioEngine class to its own file portfolio_engine.py
• Changes metrics calculations to metrics_model.py file to follow allocation_model.py and attribution_model.py
• moved unused code yahoofinance_model.py and reportlab_helpers.py to portfolio_analysis folder

How has this been tested?

python -m doctest -v openbb_terminal/portfolio/portfolio_model.py
python terminal.py ~/OpenBBTerminal/openbb_terminal/miscellaneous/scripts/portfolio/test_portfolio.openbb

[piiq]

Since we're touching docstrings here, can you run pydocstyle on the portfolio folder?

[montezdesousa]

Since we're touching docstrings here, can you run pydocstyle on the portfolio folder?

yes sir

[montezdesousa]

Since we're touching docstrings here, can you run pydocstyle on the portfolio folder?

yes sir

All good in the portfolio main menu files (except controller). The optimization I'll look in other PR will be working tday

[Chavithra]

Looks ok to me.