Update Use Cases 1.13 and 2.6 to fix error in time addition

The generation of the times array in use case 1.13 case 1 was returning an error:

library(lubridate)

# Case 1: 'time' depends on 'sdate'
repos <- '/esarchive/exp/ecmwf/system4_m1/daily_mean/$var$_f24h/$var$_$sdate$.nc'

sdates <- ymd("20010501") + rep(years(0:2), each = 1)
times <- array(ymd("20010501") + days(0:30) + rep(years(0:2), each = 31),
               dim = c(time = 31, sdate = 3))
# Error in C_time_add(out, periods, roll_month, roll_dst) : 
#   time_add: length of datetime vector must be 1 or match the length of updating vectors

This has now been changed to use sapply to generate the times array with the correct dimensions.

The data has also been updated to use a more recent system.

A similar fix was needed in use case 2.6.