Duplication of for loop code in multiple functions

[BarbaraRobson]

Most functions have similar code at the start to set things up (start and end dates, file names and locations) and then loop through the required time-steps and files, month by month or day by day. Each function has its main code inside these loops. We should be able to simplify this by having a function to do all the set-up and call a specified function passed in by name inside the loop. The R package, purrr should be useful here.