Fix subsampling bug (PolicyEngine#297)

* Fix Subsample doesn't work with behavioural response simulations PolicyEngine#296

* Versioning

* Add safety check

changelog_entry.yaml

@@ -0,0 +1,4 @@
+- bump: minor
+  changes:
+    fixed:
+    - Bug causing subsample to not work in some situations.

policyengine_core/simulations/simulation.py

@@ -1549,6 +1549,14 @@ def subsample(
         # Update the dataset and rebuild the simulation
         self.dataset = Dataset.from_dataframe(df, self.dataset.time_period)
         self.build_from_dataset()
+
+        # Ensure the baseline branch has the new data.
+        if "baseline" in self.branches:
+            baseline_tax_benefit_system = self.branches[
+                "baseline"
+            ].tax_benefit_system
+            self.branches["baseline"] = self.clone()
+            self.branches["tax_benefit_system"] = baseline_tax_benefit_system
         return self