use target currency instead of USD when computing market value

[andreasgerstmayr]

I noticed that the market value series in the cumulative value chart doesn't match my expected values. Turns out it always got converted to USD.

This PR updates the code to convert the market value to the target currency.

[erpreciso]

Thanks @andreasgerstmayr , this was on my todo list for a while: I intended to replace USD with the operating currency as defined in options. Where do you define the target_currency in your version?

[andreasgerstmayr]

Where do you define the target_currency in your version?

The target currency is the cost currency of the investment, and is set here: https://github.com/beancount/beangrow/blob/f7a9f453543a3153d0f369add1e1766c5a641697/beangrow/reports.py#L173-L178

[andreasgerstmayr]

Thanks for merging! :)