Implement JPivot's 'logical mdx' feature

[mysticfall]

From the JavaDoc API description :

/\

• JPivot by default converts MDX from a logical to a physical representation
• of the metadata after an expansion, from "all children" to a list of
• the specific children. If one wants to save the query, having the query
• represent the current state of the data, the physical state, may mean
• that the saved query will not execute at some future date because children
• and come and go. */ static final String LOGICAL_MDX_PROP = "com.tonbeller.jpivot.mondrian.logical.mdx";

[mysticfall]

Upon inspection, it seems impact of this feature on usuability is much less than I initially thought. So, I'll just clear the milestone for now to concentrate on implementing other features like exporting.