Need to implement ability to set namelist value to a config variable value

There are several namelist variables (e.g. `se_ne`) where the value of that variable is essentially the same as a particular variable in the CAM config object (e.g. `csne`). Currently a list of possible values needs to be added to the namelist definition XML file, but this can result in one missing certain possible configuration values. It also requires modifications to the namelist file whenever a new config value has been developed or added.

A better method would be to have the value of the namelist variable automatically match whatever the config variable value is. This would help avoid mismatches, and reduce long-term maintenance efforts. There are at least two possible ways to implement this:

1. Have the new ParamGen system set the namelist value to directly match the config value (although I am not 100% sure yet if that is actually doable).

2. Create a temporary environment variable that matches the config variable's value, and then have the namelist variable set to that environment variable. This is doable with either ParamGen or CIME's nmlgen system, but could potentially cause clashes with environmental variables set by CIME itself (or the other components if they also set environment variables).

Finally, alternative methods may be found as ParamGen is implemented in CAMDEN, and will be added to this issue.