[ENH] Remove yfinance as a dependency and implement data_loader

[Shuvam586]

Closes #716

• Replaces yfinance usage in notebooks with bundled static example data (stock_prices.csv and market_caps.csv).
• Adds pypfopt.data loaders for stock prices and market caps.
• Removes yfinance dependency entirely.

[Shuvam586]

@fkiraly

instead of editing tickers mentioned in the cookbook notebooks, i added 2 csv files to pypfopt/data. stock_prices.csv and market_caps.csv with only data from 2023 onwards. the stock_prices.csv is around 450kb.

also removed yfinance related statements and functions from all notebooks

[fkiraly]

Nice, thanks!

May I request to not use data downloaded via yfinance from Yahoo services at all? This is due to terms of use, we should not distribute data from Yahoo services at all in the repository or package.

Could you instead use similar data? Either completely randomly generated (Brownian motion random walk or similar, with same column names and time index), or taking some inspiration from the actual data in how you randomize - but it cannot be the exact values.