Fix do interventation

[ChristopherSchmidt89]

Do-Interventation currently only covers the case that c is parent, as implemented in distributions.py starting from
line 329 # P(e|do(c)) = \Sigma_{F} P(e|c,f) P(f)

check causaleffect package for further rules

[danthe96]

Just to add: If the code in causaleffect is too complicated, the original algorithm(s) IDC and ID are described in this paper, which causaleffect closely follows (including comments for individual lines of the algorithm). Essentially, IDC maps the conditions over into x (treatment) and y (effect) and calls ID with those parameters, which again applies some rules including kicking out non-ancestors of y etc.