Implementing shift guards that always move intervention

[nhejazi]

A proposal for a stochastic intervention that moves the natural value of the treatment as much as is possible, given the observed data, exists. Implementing such a shift requires that the ratio of the post-intervention treatment density to the empirical treatment density be evaluated and a maximum (in magnitude) shift identified for each stratum defined by the baseline covariates. An initial implementation of this is available on this branch but the efficiency of this implementation needs improvement.