Mocking real positions and calculating the corresponding return rate

[ForstMaple]

The current portfolio assumes that all assets maintain the same proportions/weights unless changed manually. This is more like the stock indices in the real market.
To mock real positions, i.e., assuming that all assets maintain the same amount unless changed manually, we need to keep each asset's market value up to date and calculate the return rate accordingly.

This function is pending further research.

[ForstMaple]

Should consider:
“cash" in a portfolio
attaching a "cost" attribute to the stock in a portfolio
changing positions with the net value inherited