<variable>
Defines a variable.
Input
- id
 - Unique variable ID
 - value
 - Variable value
 
Parent
<analysis>
Example
variable id="scale_factor" value="10e-12" />
            Comments
- Variables cannot be redefined.
 - Variables can be used with other variables to build complex strings.
 - Variables are de-references by using their name with the '@' character.
 - IDs cannot contain the '@' character.