Make subexpressions optional for constructing an AssignmentCollection

When introducing new people to pystencils it's often simpler not to
differentiate between `main_assignments` and `subexpressions` in the
beginning.
Also for simple kernels subexpressions are often not needed, since
intermediate symbols can also be set in main_assignments.

Subexpression should be kept for expert users.

pystencils/simp/assignment_collection.py

@@ -28,7 +28,7 @@ class AssignmentCollection:
     # ------------------------------- Creation & Inplace Manipulation --------------------------------------------------
 
     def __init__(self, main_assignments: Union[List[Assignment], Dict[sp.Expr, sp.Expr]],
-                 subexpressions: Union[List[Assignment], Dict[sp.Expr, sp.Expr]],
+                 subexpressions: Union[List[Assignment], Dict[sp.Expr, sp.Expr]] = {},
                  simplification_hints: Optional[Dict[str, Any]] = None,
                  subexpression_symbol_generator: Iterator[sp.Symbol] = None) -> None:
         if isinstance(main_assignments, Dict):