Backdate parameters to 2015 (#1128)

Fixes #1127

Author: nikhilwoodruff

changelog_entry.yaml

@@ -0,0 +1,4 @@
+- bump: patch
+  changes:
+    fixed:
+    - Backdated parameters to 2015 for safety.

policyengine_uk/system.py

@@ -69,6 +69,7 @@ def process_parameters(self, reform=None):
         self.parameters = uprate_parameters(self.parameters)
         self.parameters = propagate_parameter_metadata(self.parameters)
         self.add_abolition_parameters()
+        self.parameters = backdate_parameters(self.parameters, "2015-01-01")
 
         self.parameters.gov = convert_to_fiscal_year_parameters(
             self.parameters.gov