small change in schema file

schema/problem/Problem.schema

 problem:
   tree_manager:
     _type: derived
-    _base: TreeManagerBase.schema
+    _base: TreeManagerBase
     _mandatory: true
     _doc: Tree manager parameters
   dimension:
@@ -11,7 +11,7 @@ problem:
     _doc: "Dimension of the problem. '2' for 2D or '3' for 3D"
 map:
   _type: derived
-  _base: MapBase.schema
+  _base: MapBase
   _mandatory: false
   _default:
     type: MapBase
@@ -19,11 +19,11 @@ map:
   _doc: The map used in the wolf problem.
 sensors:
   _type: derived[]
-  _base: SensorBase.schema
+  _base: SensorBase
   _mandatory: true
   _doc: A sequence of all the sensors.
 processors:
   _type: derived[]
-  _base: ProcessorBase.schema
+  _base: ProcessorBase
   _mandatory: true
   _doc: A sequence of all the processors.