Allow developers to use "unset" as a value for the XML namelist generator file value field

### What is the feature/what would you like to discuss?

Since the XML file for the namelist generator, requires a value, it would be good to be able to set a value to "unset". The CCPP Framework would then generate code which sets it to the appropriate unset value.

### Is there anyone in particular you want to be part of this conversation?

@nusbaume

### Will this change (regression test) answers?

No

### Will you be implementing this enhancement yourself?

No

Locate the XML namelist generator and the CCPP Framework code that processes value fields. Trace how generated code represents unset values and determine the appropriate behavior for the literal "unset". Done means an XML value of "unset" produces generated code that assigns the appropriate unset value.

Written by the indexing model from the issue text.