Allow querying SolutionTime attribute (#146)

> SolutionTime	Float64	Solution time, in seconds

src/Interfaces/tulip_julia_api.jl

@@ -1,4 +1,5 @@
 using QPSReader
+using TimerOutputs: tottime
 
 # TODO: user-facing API in Julia
 # Other APIs should wrap this one
@@ -60,6 +61,20 @@ function get_attribute(m::Model, ::Status)
     return m.status
 end
 
+"""
+    get_attribute(model::Model, ::SolutionTime)
+
+Query the `SolutionTime` attribute from `model`
+"""
+function get_attribute(m::Model, ::SolutionTime)
+    if isnothing(m.solver)
+        return 0
+    else
+      local ns = tottime(m.solver.timer)
+      return ns * 1e-9
+    end
+end
+
 """
     get_attribute(model::Model, ::BarrierIterations)