Add Constraint Generation

[mcguenther]

Use Case:
Given is an Variability Model (VM) that may or may not be also attributed. The goal is to provide a new VM with a user-specified number of features. However, amongst these features, there should also be constraints that are similar to the original VM. Thus, we get a new VM with realistic constraints.
The resulting new VM could be attributed in the next step if the given VM was attributed.

[mcguenther]

Related: http://ec2-52-32-1-180.us-west-2.compute.amazonaws.com:8080/SPLOT/splot_fm_generator.html

also provides Constraints for features