Add support for Variable subtrees

~~_Note: This assumes a subtree with <code>variable="true"</code> attribute. <code>precondition[147-148]</code> will have to be updated when the MAT staging server updates to support variables._~~
### Notes ###
- **updated** to include <code>qdmVariable</code> parsing as the variable flag on subtrees
- pass <code>@variable</code> from within <code>data_criteria.to_model()</code>
- added missing comment to <code>precondition.handle_satisfies()</code>
- added <code>precondition.handle_precondition()</code>
  - create grouping data criteria for the variable
  - construct source data criteria from entry/data criteria
- added <code>test_with_variable</code> and associated fixture data