next_graph: (internal) move to next graph
In WY-Chen/DAGDNA: Definite Non-Ancestral Relations and Structure Learning

by finding covered edges and reverting them

next_graph(
  CIFUN,
  groot,
  POM,
  layers,
  ecount,
  h = NULL,
  gh,
  prune = T,
  verbose = F,
  conservative = T
)

`CIFUN:`	CI function
`groot:`	root DAG
`POM:`	partial ordering
`layers:`	list of layers
`ecount:`	current number of edges
`h:`	dictionary of CI statements
`gh:`	dictionary of DAGs
`prune:`	if yes, skip DAG in gh that has been checked. Otherwise not.
`convervative:`	censor by POM (aggressive) or layers (conservative) when finding covered edges