Output YAML for the Graph __repr__.

Closes #235.
Closes #291.

demes/demes.py

@@ -8,6 +8,8 @@
 
 import attr
 
+from .load_dump import dumps as demes_dumps
+
 Number = Union[int, float]
 Name = str
 Time = Number
@@ -1139,7 +1141,7 @@ def time_span(self):
         return self.start_time - self.end_time
 
 
-@attr.s(auto_attribs=True, kw_only=True, slots=True)
+@attr.s(auto_attribs=True, kw_only=True, slots=True, repr=False)
 class Graph:
     """
     The Graph class provides a high-level API for working with a demographic
@@ -1211,6 +1213,9 @@ def __contains__(self, deme_name):
         """
         return deme_name in self._deme_map
 
+    # Use the simplified YAML output for the console repr.
+    __repr__ = demes_dumps
+
     def assert_close(
         self,
         other,

demes/load_dump.py

@@ -41,8 +41,9 @@ def _load_yaml_asdict(fp):
 
 def _dump_yaml_fromdict(data, fp):
     with ruamel.yaml.YAML(typ="safe", output=fp) as yaml:
-        # Disable JSON-style inline arrays and dicts.
-        yaml.default_flow_style = False
+        # Output flow style, but only for collections that consist only
+        # of scalars (i.e. the leaves in the document tree).
+        yaml.default_flow_style = None
         # Don't emit obscure unicode, output "\Uxxxxxxxx" instead.
         # Needed for string equality after round-tripping.
         yaml.allow_unicode = False