[nixio] Special cases for annotations

Empty lists and tuples: If an annotation's value is an empty iterable,
the property value is set to empty string and the definition is used to
signify that the original value was an EMPTY LIST. On read, this is used
to set the annotation to [].
Fixes #500

Lists of strings: Multidimensional data are not supported. Lists of
strings were being detected as multidimensional because strings are
iterables. Items in an iterable are now checked against string types
before checking if they're any other type of iterable.

neo/io/nixio.py

@@ -47,6 +47,8 @@
 except NameError:
     string_types = str
 
+EMPTYANNOTATION = "EMPTYLIST"
+
 
 def stringify(value):
     if value is None:
@@ -1033,13 +1035,23 @@ def _write_property(self, section, name, v):
         elif isinstance(v, Iterable):
             values = []
             unit = None
-            if hasattr(v, "ndim") and v.ndim == 0:
+            definition = None
+            if len(v) == 0:
+                # empty list can't be saved in NIX property
+                # but we can store an empty string and use the
+                # definition to signify that it should be restored
+                # as an iterable (list)
+                values = ""
+                definition = EMPTYANNOTATION
+            elif hasattr(v, "ndim") and v.ndim == 0:
                 values = v.item()
                 if isinstance(v, pq.Quantity):
                     unit = str(v.dimensionality)
             else:
                 for item in v:
-                    if isinstance(item, pq.Quantity):
+                    if isinstance(item, string_types):
+                        item = nix.Value(item)
+                    elif isinstance(item, pq.Quantity):
                         unit = str(item.dimensionality)
                         item = nix.Value(item.magnitude.item())
                     elif isinstance(item, Iterable):
@@ -1052,6 +1064,8 @@ def _write_property(self, section, name, v):
                     values.append(item)
             section[name] = values
             section.props[name].unit = unit
+            if definition:
+                section.props[name].definition = definition
         elif type(v).__module__ == "numpy":
             section[name] = nix.Value(v.item())
         else:
@@ -1075,15 +1089,15 @@ def _nix_attr_to_neo(nix_obj):
         neo_attrs["description"] = stringify(nix_obj.definition)
         if nix_obj.metadata:
             for prop in nix_obj.metadata.props:
-                values = prop.values
-                values = list(v.value for v in values)
+                values = list(v.value for v in prop.values)
                 if prop.unit:
                     units = prop.unit
                     values = create_quantity(values, units)
                 if len(values) == 1:
-                    neo_attrs[prop.name] = values[0]
-                else:
-                    neo_attrs[prop.name] = values
+                    values = values[0]
+                if values == "" and prop.definition == EMPTYANNOTATION:
+                    values = list()
+                neo_attrs[prop.name] = values
         neo_attrs["name"] = stringify(neo_attrs.get("neo_name"))
 
         if "file_datetime" in neo_attrs: