find_covered_edges: Find covered edges
In WY-Chen/DAGDNA: Definite Non-Ancestral Relations and Structure Learning

by looking at the working DAG

1	find_covered_edges(gm, POM, layers, conservative = T)

`gm:`	a working DAG
`POM:`	partial ordering matrix. If provided, only report covered edges that agrees with ordering
`layers:`	list of layer indices. If provided, only report covered edges that agrees with ordering
`conservative:`	if yes, only use layers, do not use full POM