feat: Add portfolio.position_weights()

[y5]

solves #169.

Storing target weights inside of the portfolio seemed a bit off to me, could use an option there but I believe it's better to have that information in the user's rebalancing logic.

There were two approaches that I thought about:

• Store weights on the portfolio and keep the weights updated with each transaction & price tick
• Compute weights on the fly when needed

I've picked the second option since most applications have price ticks & transactions more frequently than accessing the portfolio weights (e.g. for rebalancing).

[avhz]

I might get @0xJepsen to chime in on this one, as they might be in progress on it.

[0xJepsen]

• Compute weights on the fly when needed

Sorry for the late response here. I think the second option is the right one.