target equity weights

[MiniXC]

It could be beneficial for many strategies to assign target weights to a number of assets, e.g. b.long(['AAPL', 'MSFT'], weights=[.5, .5]) I think sensible default behaviour would be to only allow liquidation of tickers in said weight list to achieve the target weights (but liquidation could be forced with an attribute) and that weights must add up to one. Also assets should only be rebalanced when calling long with the same set of weights again (otherwise a rebalance method could be an option as well).