fix(postprocess): Add handler for custom grid metrics

pollination/alias/inputs/postprocess.py

@@ -0,0 +1,21 @@
+from pollination_dsl.alias import InputAlias
+from queenbee.io.common import IOAliasHandler
+
+
+"""Alias for custom grid metrics used in post-processing."""
+grid_metrics_input = [
+    InputAlias.any(
+        name='grid_metrics',
+        description='A JSON file with custom metrics. This can also be a '
+        'string or a list of grid metrics.',
+        optional=True,
+        platform=['grasshopper', 'rhino', 'revit'],
+        handler=[
+            IOAliasHandler(
+                language='python',
+                module='pollination_handlers.inputs.postprocess',
+                function='grid_metrics'
+            )
+        ]
+    )
+]