Started changing the interface
Updated Cox interface

* Changed function name to coxph
* Added strata and optimizer_params parameters
* Updated help messages
* Updated user doc
* Commented out cox_prop_hazards_regr
Minor changes; started moving SQL code to Python
Moved all old SQL code into Python
Fixed bugs
Added a new Python function for compute stratification
Optimize the overall process (remove the intermediate step)
Bug fix
Clean code
Replace plpgsql with sql for the overloaded functions
Implement stratified cox
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Fixed a bug
Fixed all bugs
Finished coding
Finished all coding, install-check and user doc
deleted grouping cols
Cleanup
Fix parameter checking
Move help message to python