* 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
* 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
JosueNina