Optimizer interface and sample implementation (#19)

Reviewed-on: Rubydragon/MetagraphOptimization.jl#19
Co-authored-by: Anton Reinhard <anton.reinhard@proton.me>
Co-committed-by: Anton Reinhard <anton.reinhard@proton.me>

src/operation/find.jl

@@ -203,18 +203,18 @@ function generate_operations(graph::DAG)
     # --- find possible node fusions ---
     @threads for node in nodeArray
         if (typeof(node) <: DataTaskNode)
-            if length(node.parents) != 1
+            if length(parents(node)) != 1
                 # data node can only have a single parent
                 continue
             end
-            parent_node = first(node.parents)
+            parent_node = first(parents(node))
 
-            if length(node.children) != 1
+            if length(children(node)) != 1
                 # this node is an entry node or has multiple children which should not be possible
                 continue
             end
-            child_node = first(node.children)
-            if (length(child_node.parents) != 1)
+            child_node = first(children(node))
+            if (length(parents(child_node)) != 1)
                 continue
             end