[export, num_date] use inferred / raw_date attrs

These provide two important pieces of information for Auspice:

1. A way to unambiguously know if a tip date was inferred or not.
2. When a tip date was inferred we can now display the raw (ambiguous)
   date string.

Note that the new output would validate on the old schema as additional
properties are allowed (by default).

augur/data/schema-export-v2.json

@@ -167,6 +167,14 @@
                                         {"type": "number"},
                                         {"type": "number"}
                                     ]
+                                },
+                                "inferred": {
+                                    "type": "boolean",
+                                    "description": "[terminal nodes only] was the 'value' inferred or known?"
+                                },
+                                "raw_value": {
+                                    "type": "string",
+                                    "description": "[terminal nodes only, and only if inferred=true] the known (ambiguous) date string"
                                 }
                             }
                         },

augur/export_v2.py

@@ -859,6 +859,13 @@ def _transfer_num_date(node, raw_data):
         if is_valid(raw_data.get("num_date", None)): # it's ok not to have temporal information
             node["node_attrs"]["num_date"] = {"value": format_number(raw_data["num_date"])}
             node["node_attrs"]["num_date"].update(attr_confidence(node["name"], raw_data, "num_date"))
+            # For tips, transfer information about whether the date was inferred or known
+            # and if it was inferred then store the underlying (ambiguous) date string
+            if len(node.get('children', []))==0 and 'date_inferred' in raw_data:
+                node["node_attrs"]["num_date"]["inferred"] = raw_data['date_inferred']
+                if raw_data['date_inferred'] and raw_data.get('raw_date', False):
+                    node["node_attrs"]["num_date"]["raw_value"] = raw_data['raw_date']
+
 
     def _transfer_url_accession(node, raw_data):
         for prop in ["url", "accession"]:
@@ -918,6 +925,7 @@ def node_data_prop_is_normal_trait(name):
         'clock_length',
         'mutation_length',
         'date',
+        'date_inferred',
         'muts',
         'aa_muts',
         'sequence',