* Return weekly contracts if no standard contracts exist
* Fix unit and regression tests
* Centralize default expiration type flags
* Add ExcludeWeeklys() method
* Mark IncludeWeeklys() as obsolete since weeklies are now default
- To simplify stub generation, merge PythonSlice into Slice, following
existing pattern too, see QCAlgorithm.Python.cs
- Minor sintax fixes in example python algorithms
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* Account for premium in option strategies margin
* Minor fixes
* Add OptionInitialMargin class to separate strategies margin and premium
* Minor fix
* Minor fix
* Cleanup, docs and other minor changes
* Minor changes
* Minor changes
* Minor changes
* Address peer review
* 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`
JosueNina