Better oce printing (#329)

* Use`printstyled`

* Pretty printing for OCE

* Systematic printing

* Formatting

* Clean up comments

* Add missing test

* Update docs and export `OCESelectedParents`

Author: kahaaga

docs/src/causal_graphs.md

@@ -30,6 +30,7 @@ GraphAlgorithm
 
 ```@docs
 OCE
+OCESelectedParents
 ```
 
 ## [PC](@ref)

src/causal_graphs/causal_graphs.jl

@@ -4,6 +4,7 @@ export SimpleDiGraph
 export edges
 
 include("api.jl")
+include("printing.jl")
 
 # Concrete implementations
 include("pc/PC.jl")