Parallelise c14_calibrate_intcal()

[joeroe]

This is too slow:

library(p3k14c)
library(dplyr)
library(c14)

system.time({
  p3k14c_data |>
    mutate(cal = c14_calibrate(Age, Error))
})
#>    user  system elapsed 
#> 875.387   6.306 882.672

[joeroe]

This is better!

library(p3k14c)
library(dplyr)
library(c14)
library(future)

plan(multisession)
system.time({
  p3k14c_data |>
    mutate(cal = c14_calibrate(Age, Error))
})
#>    user  system elapsed 
#> 143.702   1.838 236.875

Although I think a lot of the time spent there is actually in do.call(what = cal), so there's still room for improvement.

The use of future::plan() is not immediately obvious, so should probably add a section of a vignette on that at some point.