with_progress(): Handle multiple consecutive progressors

[HenrikBengtsson]

Add support for something like:

with_progress({
  a <- slow_sum(1:5)
  b <- slow_sum(1:10)
  c <- slow_sum(1:15)
})

This should, by default report on three different progress sessions one after each other.

[HenrikBengtsson]

Add support for merging consecutive sessions into one, e.g. argument steps = c(5,10,15) but ideally also relative terms, e.g. weights=c(1,2,3).

[HenrikBengtsson]

library(progressr)
options(progressr.clear = FALSE)
with_progress({
  a <- slow_sum(1:3)
  b <- slow_sum(1:3)
})

gives

M: Added value 1                                                        
M: Added value 2                                                        
  |===============================================================| 100%
M: Added value 3
M: Added value 1                                                        
M: Added value 2                                                        
  |===============================================================| 100%
M: Added value 3