[Feature Request]: Universal Base Period for DoubleMLMulti

[SvenKlaassen]

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