mediation setup from `medshift`

[nhejazi]

• https://github.com/nhejazi/medshift/blob/master/R/Param_stochastic_mediation.R
• https://github.com/nhejazi/medshift/blob/master/R/LF_exptilt_ipsi.R
• https://github.com/nhejazi/medshift/blob/master/R/tmle3_Spec_medshift.R