This is the script that generates a CUPiD config file based on an existing yaml
file. I'm committing it as it was in Ingrid's cesm2_3_beta17 sandbox, for use
with CUPiD at hash b402eba; I think it will need to be updated for more recent
versions.

generate_cupid_config_file.py now relies on argparse to get a handful of
commandline arguments instead of importing some CIME functions and determining
these values on the fly (it'll be easy to set all these arguments when creating
the template)

1. Only required command line argument is cesm root directory; also allows
optional arguments for case directory (default is current dir) and cupid
example (default is key_metrics)
2. Now gets case name and short-term archive directory from CESM Case class
3. Cleaned up comment inserted at top of config.yml

generate_cupid_config_for_cesm_case.py now updates more global parameters
(start_date, end_date, climo_nyears, as well as the base_ copies) and the
time-series end_years. For now these values are hard-coded in with placeholders
until we can get them from env_cupid.xml

I thought that not specifying a default value made it required, but really it
just made the default None (which had all sorts of fun implications in the
code)

We don't know the full list of valid values when parsing command line
arguments, so this check needs to be done in the body of the python script

The header of the YAML file is a little bit bigger; in addition to mentioning
the file was auto-generated, it includes each of the three arguments
(cesm_root, case_root, and cupid_example)