Add yaml formatter parameter to allow unsafe dump

This works fine for things like PropertyList where safe_load
will read it back because of the registered representers.
If an unscrupulous user enables it for something that has not
been properly setup for yaml serialization then the subsequent
safe_load will fail on read.

python/lsst/daf/butler/configs/datastores/formatters.yaml

@@ -43,11 +43,20 @@ Packages:
   formatter: lsst.obs.base.formatters.packages.PackagesFormatter
   parameters:
     format: yaml
-PropertyList: lsst.daf.butler.formatters.yaml.YamlFormatter
-PropertySet: lsst.daf.butler.formatters.yaml.YamlFormatter
+PropertyList:
+  formatter: lsst.daf.butler.formatters.yaml.YamlFormatter
+  parameters:
+    unsafe_dump: true
+PropertySet:
+  formatter: lsst.daf.butler.formatters.yaml.YamlFormatter
+  parameters:
+    unsafe_dump: true
 NumpyArray: lsst.daf.butler.formatters.pickle.PickleFormatter
 Plot: lsst.daf.butler.formatters.matplotlib.MatplotlibFormatter
-MetricValue: lsst.daf.butler.formatters.yaml.YamlFormatter
+MetricValue:
+  formatter: lsst.daf.butler.formatters.yaml.YamlFormatter
+  parameters:
+    unsafe_dump: true
 BrighterFatterKernel: lsst.daf.butler.formatters.pickle.PickleFormatter
 StructuredDataDict: lsst.daf.butler.formatters.yaml.YamlFormatter
 Filter: lsst.obs.base.formatters.filter.FilterFormatter

python/lsst/daf/butler/formatters/yaml.py

@@ -47,6 +47,10 @@ class YamlFormatter(FileFormatter):
     unsupportedParameters = None
     """This formatter does not support any parameters"""
 
+    supportedWriteParameters = frozenset({"unsafe_dump"})
+    """Allow the normal yaml.dump to be used to write the YAML. Use this
+    if you know that your class has registered representers."""
+
     def _readFile(self, path: str, pytype: Type[Any] = None) -> Any:
         """Read a file from the path in YAML format.
 
@@ -159,7 +163,12 @@ def _toBytes(self, inMemoryDataset: Any) -> bytes:
         """
         if hasattr(inMemoryDataset, "_asdict"):
             inMemoryDataset = inMemoryDataset._asdict()
-        return yaml.safe_dump(inMemoryDataset).encode()
+        unsafe_dump = self.writeParameters.get("unsafe_dump", False)
+        if unsafe_dump:
+            serialized = yaml.dump(inMemoryDataset)
+        else:
+            serialized = yaml.safe_dump(inMemoryDataset)
+        return serialized.encode()
 
     def _coerceType(self, inMemoryDataset: Any, storageClass: StorageClass,
                     pytype: Optional[Type[Any]] = None) -> Any: