Description
Describe the feature you want to propose or implement
Add an option to set a universal base period to DoubleMLMulti as in the did package, see did documentation
Propose a possible solution or implementation
Add the option gt_combinations="universal", which sets g-anticipation-1 as base period.
Comments, context or references
Remove the exception, if the t_pre is after t_eval (required for pre-treatment periods with universal base period).
See discussion #339