Lazy derivation of gtfgco2

[bouweandela]

Partially address #42. Please check carefully if results are the same, because there are no unit tests and changes to the code are large.

[mattiarighi]

I tested this with HadGEM2-ES, but I get an error:

2019-06-18 14:10:31,501 UTC [9866] ERROR   Failed to run derive([<iris 'Cube' of surface_downward_mass_flux_of_carbon_dioxide_expressed_as_carbon / (kg m-2 s-1) (time: 48; latitude: 216; longitude: 360)>], {'short_name': 'gtfgco2', 'standard_name': '', 'long_name': 'Global Total Surface Downward CO2 Flux', 'units': 'kg s-1', 'fx_files': {'areacello': '/mnt/lustre02/work/bd0854/DATA/ESMValTool2/CMIP5_ETHZ/historical/fx/areacello/HadGEM2-ES/r0i0p0/areacello_fx_HadGEM2-ES_historical_r0i0p0.nc'}})
2019-06-18 14:10:31,542 UTC [9787] ERROR   Program terminated abnormally, see stack trace below for more information
multiprocessing.pool.RemoteTraceback: 
"""
Traceback (most recent call last):
  File "/mnt/lustre01/pf/b/b309057/SOFTWARE/miniconda3/envs/esmvaltool/lib/python3.7/multiprocessing/pool.py", line 121, in worker
    result = (True, func(*args, **kwds))
  File "/mnt/lustre01/pf/b/b309057/ESMValTool/core/esmvalcore/_task.py", line 671, in _run_task
    output_files = task.run()
  File "/mnt/lustre01/pf/b/b309057/ESMValTool/core/esmvalcore/_task.py", line 227, in run
    self.output_files = self._run(input_files)
  File "/mnt/lustre01/pf/b/b309057/ESMValTool/core/esmvalcore/preprocessor/__init__.py", line 392, in _run
    product.apply(step, self.debug)
  File "/mnt/lustre01/pf/b/b309057/ESMValTool/core/esmvalcore/preprocessor/__init__.py", line 259, in apply
    self.cubes = preprocess(self.cubes, step, **self.settings[step])
  File "/mnt/lustre01/pf/b/b309057/ESMValTool/core/esmvalcore/preprocessor/__init__.py", line 201, in preprocess
    result.append(_run_preproc_function(function, items, settings))
  File "/mnt/lustre01/pf/b/b309057/ESMValTool/core/esmvalcore/preprocessor/__init__.py", line 187, in _run_preproc_function
    return function(items, **kwargs)
  File "/mnt/lustre01/pf/b/b309057/ESMValTool/core/esmvalcore/preprocessor/_derive/__init__.py", line 111, in derive
    cube = DerivedVariable().calculate(cubes)
  File "/mnt/lustre01/pf/b/b309057/ESMValTool/core/esmvalcore/preprocessor/_derive/gtfgco2.py", line 29, in calculate
    total_flux = (fgco2_cube * area_cube).collapsed(
  File "/mnt/lustre01/pf/b/b309057/SOFTWARE/miniconda3/envs/esmvaltool/lib/python3.7/site-packages/iris/analysis/maths.py", line 411, in multiply
    in bad_coord_grps}))
ValueError: This operation cannot be performed as there are differing coordinates (time, year, day_of_month, month_number, day_of_year) remaining which cannot be ignored.
"""

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/mnt/lustre01/pf/b/b309057/ESMValTool/core/esmvalcore/_main.py", line 228, in run
    conf = main(args)
  File "/mnt/lustre01/pf/b/b309057/ESMValTool/core/esmvalcore/_main.py", line 156, in main
    process_recipe(recipe_file=recipe, config_user=cfg)
  File "/mnt/lustre01/pf/b/b309057/ESMValTool/core/esmvalcore/_main.py", line 206, in process_recipe
    recipe.run()
  File "/mnt/lustre01/pf/b/b309057/ESMValTool/core/esmvalcore/_recipe.py", line 1081, in run
    self.tasks, max_parallel_tasks=self._cfg['max_parallel_tasks'])
  File "/mnt/lustre01/pf/b/b309057/ESMValTool/core/esmvalcore/_task.py", line 596, in run_tasks
    _run_tasks_parallel(tasks, max_parallel_tasks)
  File "/mnt/lustre01/pf/b/b309057/ESMValTool/core/esmvalcore/_task.py", line 641, in _run_tasks_parallel
    task.output_files, updated_products = result.get()
  File "/mnt/lustre01/pf/b/b309057/SOFTWARE/miniconda3/envs/esmvaltool/lib/python3.7/multiprocessing/pool.py", line 657, in get
    raise self._value
ValueError: This operation cannot be performed as there are differing coordinates (time, year, day_of_month, month_number, day_of_year) remaining which cannot be ignored.
2019-06-18 14:10:31,549 UTC [9787] INFO    If you suspect this is a bug or need help, please open an issue on https://github.com/ESMValGroup/ESMValTool/issues and attach the run/recipe_*.yml and run/main_log_debug.txt files from the output directory.

[bouweandela]

I think this derivation function is no longer needed, as this derived variable can be computed from fgco2 using the preprocessor:

preprocessors:
  global_total:
    area_statistics:
      operator: sum
      fx_files: [areacello]

@ledm Can you comment on this?