FEA generator function for m-separating sets

[adam2392]

Is your feature request related to a problem? Please describe.
A generator for m-separating sets from Van Zander 2015.

Describe the solution you'd like
Given an ancestral graph (CPDAG/MAG/PAG), generate in polynomial-delay time separators and adjustment sets (see adjustmentSets in dagitty R package). This would be a function that yields a new m-separating set.

def m_separating_sets(graph, source, target):
    … implementation details
    yield separating_set

See the API for this kind of generators in networkx: https://networkx.org/documentation/stable/reference/algorithms/generated/networkx.algorithms.shortest_paths.generic.all_shortest_paths.html#networkx.algorithms.shortest_paths.generic.all_shortest_paths

Additional context
py-why/dowhy#402 (comment)

cc: @aryan26roy