Add GraphProperties and property caching

src/MetagraphOptimization.jl

@@ -25,7 +25,7 @@ export is_exit_node
 export parents
 export children
 export compute
-export graph_properties
+export get_properties
 export get_exit_node
 export is_valid
 
@@ -55,6 +55,8 @@ export bytes_to_human_readable
 import Base.length
 import Base.show
 import Base.==
+import Base.+
+import Base.-
 import Base.in
 import Base.copy
 import Base.isempty
@@ -66,6 +68,7 @@ import Base.collect
 include("task/type.jl")
 include("node/type.jl")
 include("diff/type.jl")
+include("properties/type.jl")
 include("operation/type.jl")
 include("graph/type.jl")
 
@@ -96,6 +99,9 @@ include("operation/get.jl")
 include("operation/print.jl")
 include("operation/validate.jl")
 
+include("properties/create.jl")
+include("properties/utility.jl")
+
 include("task/create.jl")
 include("task/compare.jl")
 include("task/print.jl")