Make Sample abstract; create MemorySample

tests/components/test_static_lazy.py

@@ -11,7 +11,7 @@ from numpy.testing import assert_array_equal
 from miplearn.classifiers import Classifier
 from miplearn.classifiers.threshold import Threshold, MinProbabilityThreshold
 from miplearn.components.static_lazy import StaticLazyConstraintsComponent
-from miplearn.features.sample import Sample
+from miplearn.features.sample import Sample, MemorySample
 from miplearn.instance.base import Instance
 from miplearn.solvers.internal import InternalSolver, Constraints
 from miplearn.solvers.learning import LearningSolver
@@ -22,7 +22,7 @@ from miplearn.types import (
 
 @pytest.fixture
 def sample() -> Sample:
-    sample = Sample(
+    sample = MemorySample(
         {
             "constr_categories": [
                 "type-a",