Implement Index Rebalance

[sadhbh-c0d3]

• Create Delta Short vector between new asset weights and old weights, and execute as Buy order
• Create Delta Long vector between new asset weights and old weights, and execute as Sell order
• Should new rebalance happen before old completed, then update Delta Short/Long with change of weights
• Conclusion: each Index should always carry rebalace Short/Long vectors, and they should be 0 unless there is rebalance
• Keeper to "poke" the process forward