* Add support for Null SecurityPositionGroupModel
- Add support for algorithms to set the null security position group
model which will not group securities together but rather return the
single group buying power model. Adding regression algorithms
* Add Null Margin python regression & cleanup
- Add python regression algorithm & minor regression algorithms clean up
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* Handle liquidating option position with an option strategy
* Handle liquidating option position with an option strategy
* Minor changes
* Minor fix in default margin model margin call order generation