Finish `CoreSet2` constructor method

[ChristopherEeles]

• [x] The CoreGx2 constructor will be used temporarily from the Fall 2021 to Spring 2022 releases to ensure backwards compatibility
• [x] Propagate the constructor changes into dependent packages
• [ ] Once the new constructor is implemented, we can build PSets from the drug combo data Nicholas Li has been working on
• [ ] These PSets will be used to update the summary and signature methods
• [ ] Finally, we will need to refactor the monotherapy PSet creation scripts to use the new constructor
• [ ] A deprecation warning will be added to CoreSet and other first version constructor
• [ ] For the Spring 2022 release, we will swap CoreSet with CoreSet2, so they main constructor uses the new method
• [ ] Finally, we will remove CoreSet2 in the Fall 2022 release.