Covariances when adding product terms

[jhorzek]

When adding product terms in case of linear = FALSE and use_definition_variables = FALSE, (co-) variances of all exogenous variables are automatically added. These are not freely estimated but set to the observed covariances. This should, in theory, be identical to an estimation. Limitations:

• [x] It is currently unclear if covariances between the residuals of x1, x2, ..., y1, y2, ... and the product terms should be added. These would have to be estimated, considerably increasing the number of model parameters
• [ ] Thoroughly test the current approach
• [x] Adapt tests: We are now estimating fewer parameters

[jhorzek]

We will keep adding product terms as an option, but default to using definition variables and OpenMx. For the time being, there is no further development of the product approach.