Dynamically update forcing paths in model driver

For longer running simulations (such as spinup), it is a common use case
to repeat the forcing over however many years of forcing we have
throughout a model run.

This change modifies the CABLE driver to dynamically update the forcing
paths in the CABLE namelist file when an optional config file
(cable.forcing_year.yaml) is present in the experiment directory. The
optional configuration file can be used to specify which years of
forcing should be run and or repeated via the `offset` and `repeat`
keys.

This was originally inside a setup script, however we decided this is
better off being implemented within the driver as: 1. the script was
duplicating a lot of the logic in payu when inferring the current
simulation year, and 2. running python in a setup script requires the
user to have the correct python environment activated which may be prone
to errors - implementing the script in the driver avoids this issue.

payu/models/cable.py

@@ -8,6 +8,7 @@
 """
 
 # Standard Library
+import glob
 import os
 import shutil
 
@@ -36,7 +37,39 @@ def __init__(self, expt, name, config):
             'pft_params.nml',
         ]
 
-        self.restart_info = {}
+        self.forcing_year_config = 'cable.forcing_year.yaml'
+        self.optional_config_files = [self.forcing_year_config]
+
+        self.met_forcing_vars = [
+            "Rainf",
+            "Snowf",
+            "LWdown",
+            "SWdown",
+            "PSurf",
+            "Qair",
+            "Tair",
+            "Wind",
+        ]
+
+    def _get_forcing_path(self, variable, year):
+        """Return the met forcing file path for a given variable and year."""
+        pattern = os.path.join(self.work_input_path, f"*{variable}*{year}*.nc")
+        for path in glob.glob(pattern):
+            return path
+        msg = f"Unable to infer met forcing path for variable {variable} for year {year}."
+        raise FileNotFoundError(msg)
+
+    def _update_forcing(self, year, offset=None, repeat=None):
+        """Update the CABLE namelist file to use the correct met forcing."""
+        if offset:
+            year += offset[1] - offset[0]
+        if repeat:
+            year = repeat[0] + ((year - repeat[0]) % (repeat[1] - repeat[0] + 1))
+        for var in self.met_forcing_vars:
+            path = self._get_forcing_path(var, year)
+            self.cable_nml["cable"]["gswpfile"][var] = (
+                os.path.relpath(path, start=self.work_path)
+            )
 
     def set_model_pathnames(self):
         super(Cable, self).set_model_pathnames()
@@ -61,9 +94,16 @@ def setup(self):
         if self.prior_restart_path:
             with open(self.restart_calendar_path, 'r') as restart_file:
                 self.restart_info = yaml.safe_load(restart_file)
-            self.cable_nml['cable']['ncciy'] = self.restart_info['year']
         else:
-            self.restart_info['year'] = self.cable_nml['cable']['ncciy']
+            self.restart_info = {'year': self.cable_nml['cable']['ncciy']}
+
+        year = self.cable_nml['cable']['ncciy'] = self.restart_info['year']
+
+        forcing_year_config_path = os.path.join(self.work_path, self.forcing_year_config)
+        if os.path.exists(forcing_year_config_path):
+            with open(forcing_year_config_path, 'r') as file:
+                forcing_year_config = yaml.safe_load(file)
+            self._update_forcing(year, **forcing_year_config)
 
         # Write modified namelist file to work dir
         self.cable_nml.write(