Add power analysis for the different models

[cedricbatailler]

People should be able to conduct power analysis in JSmediation.

1. As much as we can, the logic to conduct mediation analysis should be as close as possible to the logic to conduct mediation.
2. Underlying assumptions regarding the power analysis should be explicit
3. If possible, the framework should make it easy to conduct sensibility analysis for some parameters to see what are the consequences of researchers' assumptions