Add new DEBUG variable

[gsketefian]

DESCRIPTION OF CHANGES:

1. Add a new experiment configuration variable named DEBUG to enable more in-depth debugging output from workflow scripts. Set default value of DEBUG in config_defaults.sh to "FALSE".
2. In experiment generation scripts, change circumstances under which different messages are printed to screen (e.g. when VERBOSE is "TRUE", when DEBUG is "TRUE", or always).
3. In experiment generation scripts, for clarity add new informational messages and modify some existing ones.
4. In various scripts, change "set -x" to "set +x" to reduce output clutter. This can be changed back as necessary (e.g. for debugging).

Note that if DEBUG is set to "TRUE", VERBOSE will get reset to "TRUE" if necessary in order to also print out all the VERBOSE messages.

TESTS CONDUCTED:

Ran the WE2E test grid_RRFS_CONUS_25km_ics_FV3GFS_lbcs_FV3GFS_suite_GFS_v15p2 as-is as well as with modifications to the default values of VERBOSE and DEBUG, as follows:

1. grid_RRFS_CONUS_25km_ics_FV3GFS_lbcs_FV3GFS_suite_GFS_v15p2 as-is, i.e. using default values VERBOSE="TRUE" and DEBUG="FALSE".
2. grid_RRFS_CONUS_25km_ics_FV3GFS_lbcs_FV3GFS_suite_GFS_v15p2 modified with VERBOSE="FALSE" (and with default of DEBUG="FALSE").
3. grid_RRFS_CONUS_25km_ics_FV3GFS_lbcs_FV3GFS_suite_GFS_v15p2 modified with DEBUG="TRUE" (and with default of VERBOSE="TRUE").
4. grid_RRFS_CONUS_25km_ics_FV3GFS_lbcs_FV3GFS_suite_GFS_v15p2 modified with DEBUG="TRUE" and VERBOSE="FALSE" (which should get reset to "TRUE").

All tests were successful. The experiment generation log files (log.generate_FV3LAM_wflow.sh) were compared and differed in the expected ways.

DOCUMENTATION:

Necessary documentation of DEBUG is in config_defaults.sh. Created Issue #640 to also update rst documentation.