fix(phenomena): set default value of _schema to None for proper initialization

Author: phillipus85

src/pydasa/workflows/phenomena.py

@@ -79,7 +79,7 @@ class AnalysisEngine(Foundation):
     """Dictionary of all dimensional variables in the problem."""
 
     # :attr: _schema
-    _schema: Optional[FDUs] = Frameworks.PHYSICAL.value
+    _schema: Optional[FDUs] = None
     """Dimensional framework schema (manages FDUs). Always a Schema object after initialization."""
 
     # :attr: _model