[DelayedSample(Set)] make load and delayed_attributes private

This removes the need for a lot of guessing in downstream packages as they can start removing all keys that start with _ when access of the sample's attribute is needed.

bob/pipelines/tests/test_samples.py

@@ -101,6 +101,10 @@ def test_delayed_samples():
     child_sample = Sample(1, parent=delayed_sample)
     assert child_sample.data == 1, child_sample.data
     assert child_sample.annot == "annotation", child_sample.annot
+    assert child_sample.__dict__ == {
+        "data": 1,
+        "annot": "annotation",
+    }, child_sample.__dict__
 
     delayed_sample.annot = "changed"
     assert delayed_sample.annot == "changed", delayed_sample.annot