Create Portfolio class; Add, withdraw, and get cash
Buy stocks and mutual funds
Define Assett, Stock, and MutualFund classes
Add history and gatherAssets functions
Add sellStock function
Aggregated buyStock and buyMutualFund functions into general buyAsset function
Aggregated sellStock and sellMutualFund into sellAsset function
Fixed sellAsset function
Modified buyAsset and sellAsset functions