tutorial: isodatetime

[oliver-sanders]

Introduce the isodatetime command into the tutorial (note this is currently done in the Rose tutorial).

The forecast task must access data from tasks in earlier cycles, this is a good opportunity to introduce the isodatetime command.

E.G. something like:

[runtime]
  [[forecast]]
    ..
    [[[environment]]]
      FORECAST_TIME = $CYLC_TASK_CYCLE_POINT
      PREV_FORECASTS = """
          $(isodatetime $CYLC_TASK_CYCLE_POINT --offset=PT6H)
          $(isodatetime $CYLC_TASK_CYCLE_POINT --offset=PT12H)
          $(isodatetime $CYLC_TASK_CYCLE_POINT --offset=PT18H)
      """