[develop]: Source task specific portions of var_defns.sh

[danielabdi-noaa]

DESCRIPTION OF CHANGES:

Adds capability to source task specific sections of var_defns.sh file. This has some pros and cons, so I am not sure if it is a good idea yet.

• pro: Each task sources a subset of the environment variables it needs defined for it in config_defaults.yaml, along with common variables defined for all tasks. Different tasks could define the same variable name e.g. NNODES, instead of NNODES_FOR_FCST, NNODES_FOR_POST etc. So it is a good way to handle proliferation of tasks.

• con: Some variables could be shared by multiple tasks, a developer could start using a variable defined for make_grid in run_fcst etc. This maintainability issue is the main reason I don't like this approach and why I wanted to put it in a separate PR. Either the variable has to be put in a shared region, or the given task need to be told to use variables from the other task.
  For example, make_orog uses lots of variables defined for make_grid in the current structure of config_defaults.yaml, so I have to do the following in exregional_make_orog.sh:

source_config_for_task "task_make_orog|task_make_grid" ${GLOBAL_VAR_DEFNS_FP}

I am not convinced that this PR is a good idea so please let me know what you think about it.
The last commit has the changes relevant for this PR.

TESTS CONDUCTED:

I have run tests successfully on HERA

No failures on the following 56 tests

• community_ensemble_008mems
• community_ensemble_2mems
• deactivate_tasks
• grid_CONUS_25km_GFDLgrid_ics_FV3GFS_lbcs_FV3GFS_suite_GFS_v16
• grid_CONUS_3km_GFDLgrid_ics_FV3GFS_lbcs_FV3GFS_suite_GFS_v16
• grid_RRFS_AK_13km_ics_FV3GFS_lbcs_FV3GFS_suite_GFS_v16
• grid_RRFS_AK_3km_ics_FV3GFS_lbcs_FV3GFS_suite_GFS_v16
• grid_RRFS_CONUS_13km_ics_FV3GFS_lbcs_FV3GFS_suite_GFS_v15p2
• grid_RRFS_CONUS_13km_ics_FV3GFS_lbcs_FV3GFS_suite_GFS_v16
• grid_RRFS_CONUS_25km_ics_FV3GFS_lbcs_FV3GFS_suite_GFS_2017_gfdlmp
• grid_RRFS_CONUS_25km_ics_FV3GFS_lbcs_FV3GFS_suite_GFS_2017_gfdlmp_regional
• grid_RRFS_CONUS_25km_ics_FV3GFS_lbcs_FV3GFS_suite_GFS_v15p2
• grid_RRFS_CONUS_25km_ics_FV3GFS_lbcs_FV3GFS_suite_GFS_v16
• grid_RRFS_CONUS_25km_ics_FV3GFS_lbcs_RAP_suite_HRRR
• grid_RRFS_CONUS_25km_ics_GSMGFS_lbcs_GSMGFS_suite_GFS_2017_gfdlmp
• grid_RRFS_CONUS_25km_ics_GSMGFS_lbcs_GSMGFS_suite_GFS_v15p2
• grid_RRFS_CONUS_25km_ics_NAM_lbcs_NAM_suite_HRRR
• grid_RRFS_CONUS_25km_ics_NAM_lbcs_NAM_suite_RRFS_v1beta
• grid_RRFS_CONUS_3km_ics_FV3GFS_lbcs_FV3GFS_suite_GFS_v15p2
• grid_RRFS_CONUS_3km_ics_FV3GFS_lbcs_FV3GFS_suite_GFS_v15_thompson_mynn_lam3km
• grid_RRFS_CONUS_3km_ics_FV3GFS_lbcs_FV3GFS_suite_GFS_v16
• grid_RRFS_CONUScompact_13km_ics_FV3GFS_lbcs_FV3GFS_suite_GFS_v16
• grid_RRFS_CONUScompact_13km_ics_HRRR_lbcs_RAP_suite_HRRR
• grid_RRFS_CONUScompact_13km_ics_HRRR_lbcs_RAP_suite_RRFS_v1alpha
• grid_RRFS_CONUScompact_13km_ics_HRRR_lbcs_RAP_suite_RRFS_v1beta
• grid_RRFS_CONUScompact_25km_ics_FV3GFS_lbcs_FV3GFS_suite_GFS_v16
• grid_RRFS_CONUScompact_25km_ics_HRRR_lbcs_HRRR_suite_HRRR
• grid_RRFS_CONUScompact_25km_ics_HRRR_lbcs_HRRR_suite_RRFS_v1beta
• grid_RRFS_CONUScompact_25km_ics_HRRR_lbcs_RAP_suite_HRRR
• grid_RRFS_CONUScompact_25km_ics_HRRR_lbcs_RAP_suite_RRFS_v1alpha
• grid_RRFS_CONUScompact_25km_ics_HRRR_lbcs_RAP_suite_RRFS_v1beta
• grid_RRFS_CONUScompact_3km_ics_FV3GFS_lbcs_FV3GFS_suite_GFS_v16
• grid_RRFS_CONUScompact_3km_ics_HRRR_lbcs_RAP_suite_GFS_v15p2
• grid_RRFS_CONUScompact_3km_ics_HRRR_lbcs_RAP_suite_HRRR
• grid_RRFS_CONUScompact_3km_ics_HRRR_lbcs_RAP_suite_RRFS_v1alpha
• grid_RRFS_CONUScompact_3km_ics_HRRR_lbcs_RAP_suite_RRFS_v1beta
• grid_RRFS_NA_13km_ics_FV3GFS_lbcs_FV3GFS_suite_RRFS_v1beta
• grid_RRFS_SUBCONUS_3km_ics_FV3GFS_lbcs_FV3GFS_suite_GFS_v16
• grid_RRFS_SUBCONUS_3km_ics_HRRR_lbcs_RAP_suite_GFS_v15p2
• grid_SUBCONUS_Ind_3km_ics_FV3GFS_lbcs_FV3GFS_suite_GFS_v16
• grid_SUBCONUS_Ind_3km_ics_HRRR_lbcs_RAP_suite_HRRR
• grid_SUBCONUS_Ind_3km_ics_HRRR_lbcs_RAP_suite_RRFS_v1beta
• inline_post
• MET_ensemble_verification
• MET_verification
• nco_ensemble
• nco_grid_RRFS_CONUScompact_25km_ics_HRRR_lbcs_RAP_suite_HRRR
• custom_ESGgrid
• custom_GFDLgrid
• custom_GFDLgrid__GFDLgrid_USE_GFDLgrid_RES_IN_FILENAMES_eq_FALSE
• custom_GFDLgrid__GFDLgrid_USE_GFDLgrid_RES_IN_FILENAMES_eq_TRUE
• pregen_grid_orog_sfc_climo
• specify_DOT_OR_USCORE
• specify_DT_ATMOS_LAYOUT_XY_BLOCKSIZE
• specify_RESTART_INTERVAL
• specify_template_filenames

DEPENDENCIES:

• [develop]: Use yaml format for specifying experiment parameters #778
• [develop]: Remove shell workflow. #764

DOCUMENTATION:

None

ISSUE (optional):

None

CONTRIBUTORS (optional):

@christinaholtNOAA