Add tests

src/MetagraphOptimization.jl

@@ -15,6 +15,8 @@ export AbstractComputeTask
 export AbstractDataTask
 export DataTask
 export FusedComputeTask
+export PossibleOperations
+export GraphProperties
 
 export make_node
 export make_edge

src/properties/utility.jl

@@ -52,6 +52,7 @@ function -(prop::GraphProperties)
         data = -prop.data,
         computeEffort = -prop.computeEffort,
         computeIntensity = prop.computeIntensity,   # no negation here!
+        cost = -prop.cost,
         noNodes = -prop.noNodes,
         noEdges = -prop.noEdges,
     )::GraphProperties