Add Prior knowledge for LiM #145

[yujota]

Hi there. Thank you for sharing wonderful lib.

I implemented prior knowledge for LiM, related to issue #145.

Still, I'm wondering if this implementation is theoretically ok. I look forward to your suggestion.

[YanaZeng]

Dear yujota,

Thanks for implementing prior knowledge for LiM. The modified codes look feasible and ok.

But please note that LiM optimizes the adjacency matrix about the directed EDGES. Element "1" in the matrix means the existence of the directed edge, other than the directed PATH.

Thank you again! Best, Yana