Change environment.Closer interface

Changed the environment.Closer interface so that the `Close` method
return an error.

environment/Environment.go

@@ -55,7 +55,7 @@ type PixelEnvironment interface {
 // Closer is an environment which can be closed
 type Closer interface {
 	Environment
-	Close()
+	Close() error
 }
 
 // RowColer is an environment that can return the rows and columns of

environment/gym/Gym.go

@@ -158,3 +158,10 @@ func (g *GymEnv) End(*ts.TimeStep) bool {
 func (g *GymEnv) AtGoal(mat.Matrix) bool {
 	panic("atGoal: cannot calculate at goal for GymEnv")
 }
+
+// Close performs resource cleanup after the environment is no longer
+// needed
+func (g *GymEnv) Close() error {
+	g.Environment.Close()
+	return nil
+}

environment/mujoco/internal/mujocoenv/MujocoEnv.go

@@ -371,9 +371,11 @@ func (m *MujocoEnv) ActionSpec() environment.Spec {
 
 // Close performs cleanup of MuJoCo resources once the environment
 // is no longer needed.
-func (m *MujocoEnv) Close() {
+func (m *MujocoEnv) Close() error {
 	C.mj_deleteModel(m.Model)
 	C.mj_deleteData(m.Data)
+
+	return nil
 }
 
 // ID returns the ID of object with name name