Convert property value nan to NaN.

https://gitlab.com/caosdb/caosdb-pylib/-/merge_requests/60

src/caosdb/common/models.py

@@ -856,6 +856,8 @@ class Entity(object):
                     xml.append(v_elem)
             elif self.value == "":
                 xml.append(etree.Element("EmptyString"))
+            elif str(self.value) == "nan":
+                xml.text = "NaN"
             else:
                 xml.text = str(self.value)