frollapply simplifylist could be more smart about median results

```r
simplifylist(list(
  NA, median(1:2), median(1:3)
), fill=NA, ansmask=c(F,T,T)
) -> a
str(a)
#List of 3
# $ : logi NA
# $ : num 1.5
# $ : int 2
```
this should be simplified to handle adaptive rolling median well