src(gates): use cattrs

Author: zbjornson

.vscode/settings.json

@@ -3,5 +3,7 @@
 		"cellengine",
 		"cytometry",
 		"scaleset"
-	]
+	],
+	"python.linting.enabled": true,
+	"python.linting.flake8Enabled": true
 }

CONTRIBUTING.md

@@ -0,0 +1,22 @@
+## Contributing
+
+### Running tests
+
+Unit tests run with `python3 -m pytest`.
+
+The integration tests need CellEngine credentials to run. In CI, these are
+provided by GitHub secrets. Locally, you need to set environment variables:
+
+```sh
+CELLENGINE_USERNAME="..."
+CELLENGINE_PASSWORD="..."
+# for running tests that require S3:
+S3_ACCESS_KEY="..."
+S3_SECRET_KEY="..."
+# Run the integration tests:
+python3 -m pytest ./tests/integration/test_integration.py
+```
+
+### Conventions
+
+* Error message text should end with a period.