* Adds NullBuyingPowerModel
The `NullBuyingPowerModel` considers that we have sufficient buying power for all orders.
Adds example using a bull call spread since a equity buy and hold would not show the impact of this model in the position group buing power model.
* Define NullBuyingPowerModel.GetMaintenanceMargin
The `NullBuyingPowerModel.GetMaintenanceMargin` returns a `MaintenanceMargin` of zero. It means the total margin used is always zero and margin calls will not be triggered.
This feature is inspired on the `ConstantBuyingPowerModel`. We don't inherit from that class, because `ConstantBuyingPowerModel.GetInitialMarginRequirement` gives us very small initial margin that leads to a very large quantity if we use `SetHoldings` or `CalculateOrderQuantity`
Alexandre Catarino