This function is used to filter out everything except a specified subset of
the data. The each parameter is used to change slice's behaviour to filter
out all except a specified subset within each group or, if no grouping,
within each node.
Usage
slice(.self, rows = NULL, start = NULL, end = NULL, each = FALSE, auto_compact = NULL)
dat <- Multiplyr (x=1:100, G=rep(c("A", "B", "C", "D"), each=25))
dat %>% group_by (G)
dat %>% slice (1:10, each=TRUE)
dat %>% slice (1:10)
dat %>% shutdown()