Improve performance of the calculation of some metrics for large-scale feature models

[jmhorcas]

Some operations takes too much computation time to be calculated for large-scale feature models.
Those metrics are related to the "depth tree" and the "constraints per features" metrics.

[jmhorcas]

This is because of the transformations of the cross-tree constraints to CNF.
This needs to be changed in this issue in Flama