Make default implementations of to_model, etc generate an error

src/instance.jl

@@ -5,9 +5,17 @@
 using JSON2
 import Base: dump
 
-get_instance_features(instance) = [0.]
-get_variable_features(instance, var, index) = [0.]
+to_model(instance) =
+    error("not implemented: to_model")
+
+get_instance_features(instance) =
+    error("not implemented: get_instance_features")
+
+get_variable_features(instance, var, index) =
+    error("not implemented: get_variable_features")
+
 find_violated_lazy_constraints(instance, model) = []
+
 build_lazy_constraint(instance, model, v) = nothing
 
 dump(instance::PyCall.PyObject, filename) = @pycall instance.dump(filename)

test/knapsack.jl

@@ -2,7 +2,8 @@
 #  Copyright (C) 2020, UChicago Argonne, LLC. All rights reserved.
 #  Released under the modified BSD license. See COPYING.md for more details.
 
-import MIPLearn: get_instance_features,
+import MIPLearn: to_model,
+                 get_instance_features,
                  get_variable_features
                  find_violated_lazy_constraints
 using JuMP
@@ -26,7 +27,6 @@ function get_instance_features(data::KnapsackData)
     return [0.]
 end
 
-
 function get_variable_features(data::KnapsackData, var, index)
     return [0.]
 end