stochastic interventional (in)direct effects

[nhejazi]

As of 3e65185, support for the stochastic interventional effects (see methods manuscript at https://arxiv.org/abs/2009.06203) is included in a helper package, https://github.com/nhejazi/medshift/tree/master/sandbox/intmedlite. These estimators can be integrated into the package with only relatively minor changes to the code; however, for accessibility, this integration should be prioritized.