Add Intervention Distribution to Information Flow

[dglmoore]

Description

To properly discuss causal structure, we need to be able to apply generic intervention distributions. The current implementation of inform_effective_info admits this possibility, but inform_info_flow does not. We should rectify this situation.

Proposed Resolution

EXPORT double inform_information_flow(int const *src, int const *dst,
    int const *back, double const *inter, size_t l_src, size_t l_dst,
    size_t l_back, size_t n, size_t m, int b, inform_error *err);