add sumrows to valid field list for st_new

[kylebaron]

sumrows needs to be in the list of valid incoming fields when calling st_new()

library(dplyr)
library(pmtables)

pt_cont_long(
  pmt_first,
  cols = c("WT", "CRCL"),
  by  = "STUDYf"
) %>% st_new()