[tern.mmrm] enhancement ideas for LS means interface / extraction

this is a feature request

currently the lsmeans comparison is only between two arms (ARM|VISIT), no covariates is supported. However the ls means needed is REGION*ARM|VISIT. I wonder if the comparison for lsmeans could be specified

Another request is that sometimes even the model fails to converge we also need that result. Currently it is raising error on such situations, I wonder if an argument like "ignore_warning = TRUE" to use that result despite the warings

Another request is that in t_mmrm I wonder if we can filter out the groups we needed. Sometimes we only want to show part of the comparisons, e.g. here we only want the China sub population result.

Provenance:
```
Creator: lil128
```