Draft: Develop multiple steps

This is some progress of multiple steps. It is just one idea, but there may be many ways to build this development. Here is a script and you can see the planned structure.

  repos <- "/esarchive/exp/ecmwf/system5_m1/monthly_mean/tas_f6h/$var$_$sdate$.nc"
  var <- 'tas'
  sdate <- c('20170101', '20170201')
  data <- Start(dat = repos,
                var = var,
                sdate = c('20170101'), #, '20170401'),
                ensemble = 'all', #indices(1),
                time = 'all', #indices(1), 
                latitude = indices(1:50), #'all',
                longitude = indices(1:100),  #'all',
                return_vars = list(time = c('sdate'), 
                                   longitude = NULL, latitude = NULL),
                retrieve = FALSE
               )

  # fun1: add noise to time
  fun1 <- function(x) {
    # [time]
    x + rnorm(length(x))
  }

  # fun2: average over ensemble
  fun2 <- function(x) {
    # [ensemble]
    mean(x)
  }

  step1 <- Step(fun1, target_dims = c('time'),
                      output_dims = c('time'))
  step2 <- Step(fun2, target_dims = c('ensemble'),
                      output_dims = NULL)

  wf <- AddStep(inputs = data, step_fun = list(step_a = step1, step_b = step2))

  #NOT WORKING YET
  res <- Compute(wf,
                 chunks = list(step_a = list(ensemble = 2),
                               step_b = list(time = 2)))