Bugfix in I/O for user-defined concrete graph types: add getter `prop…

…erties` and setter `set_properties!` to AbstractGraph interface

src/computational_graph/abstractgraph.jl

@@ -89,6 +89,13 @@ function operator(g::AbstractGraph) end
 """
 function weight(g::AbstractGraph) end
 
+"""
+    function properties(g::AbstractGraph)
+
+    Returns additional properties, if any, of the computational graph `g`.
+"""
+function properties(g::AbstractGraph) end
+
 """
 function subgraph(g::AbstractGraph, i=1)
     
@@ -184,6 +191,13 @@ function set_weight!(g::AbstractGraph, weight)
 """
 function set_weight!(g::AbstractGraph, weight) end
 
+"""
+function set_properties!(g::AbstractGraph, properties)
+
+    Update the properties of graph `g` to `properties`.
+"""
+function set_properties!(g::AbstractGraph, properties) end
+
 """
 function set_subgraph!(g::AbstractGraph, subgraph::AbstractGraph, i=1)