small fixes

Author: erikvansebille

parcels/field.py

@@ -37,7 +37,7 @@
 
 
 def _isParticle(key):
-    if hasattr(key, "obs_written"):
+    if hasattr(key, "_data"):
         return True
     else:
         return False

parcels/particleset.py

@@ -227,7 +227,7 @@ def lonlatdepth_dtype_from_field_interp_method(field):
 
     @property
     def size(self):
-        return (len(self._data["trajectory"]), len(self._data["obs"]))
+        return len(self._data["trajectory"])
 
     @property
     def pclass(self):