add ability to specify namelist requirements via xml

**Bill Sacks < sacks@ucar.edu > - 2014-06-06 15:27:44 -0600**
**Bugzilla Id:** [1989](http://bugz.cgd.ucar.edu/show_bug.cgi?id=1989)
**Bugzilla CC:** andre@ucar.edu, muszala@ucar.edu, mvertens@ucar.edu, rfisher@ucar.edu,

Erik and I were talking about how it would be nice to be able to specify namelist requirements via an xml file, in order to simplify the error-checking that we currently do in build_namelist.

We could see three possible ways to do this:

(1) Specify requirements (somehow) in the namelist definition xml file

(2) Have a file similar to namelist_defaults, but giving requirements rather than defaults

(3) If a lot of our error-checking is along the lines of:

If variable X=xxx, then variable Y should be yyy, which is its default value

- Then we could implement this by having something like an "unmodifiable" attribute in the namelist_defaults file.

e.g., rather than

yyy

instead, have:

yyy

If the unmodifiable attribute is set, that means that, not only is this its default value, but also it cannot be modified from this value by the user.

This could possibly be implemented in the add_default call.