Add 'tidy' methods for Tplyr tables

[elimillera]

When programming a table it would be nice to use filter to call filter.tplyr_table to be able to add filters or 'where' clauses on before or after the build.

Example

tplyr_table(adsl, TRT01P) %>% 
  add_layer(
    group_desc(AGE, by = "Age (years)")
  ) %>% 
  filter(SAFFL == "Y") %>%
  build()