AE specification subgroup analysis

[elong0527]

After looking the code in prepare_ae_specific_subgroup, I feel we need to talk about the function design @wangben718

1. Is it possible to use extend_ae_specific_subgroup? that basically add an indicator to rerun prepare_ae_specific + extend_ae_specific + format_ae_specific in each subgroup?

extend_ae_specific_subgroup <- function(outdata, subgroup){
     outdata$subgroup <- subgroup 

    outdata
}

2. Create an internal function called obtain_ae_specifc_subgroup. If outdata$subgroup is not null, we will rerun the code in each subgroup.

3. Create an internal function called tlf_ae_specific_subgroup. If outdata$subgroup is not null, we will generate the TLFs using this internal function if user called tlf_ae_sepcific.

Benefit:

1. all extended_ae_specific functions can be used
2. can potentially extend the framework for ae_summary analysis in the future.