General Updates
* implemented the multi-level variables list model (for crosstabs)
* implemented the Crosstabs UI
Implementing the linear regression options and UI
Removal of contingency tables

They’re called Crosstabs
Created options and UI for correlation analysis
Small fixes to linear regression
Update to the display of tables

Now a nested list of tables will display properly.
Created options and UI for partial correlation analysis
Added initial SEM analysis options and UI
Rudimentary SEM implementation
Merge pull request #16 from SachaEpskamp/master

Rudimentary SEM implementation