Implement __bool__ for AssignmentCollection

AssignmentCollection can be used in many cases where you could also use
a List[Assignment]. With AssignmentCollection.__bool__, an empty
AssignmentCollection is falsy and a non-empty one truthy.

So you can `assert assignments, 'must not be emtpy'`

pystencils/simp/assignment_collection.py

@@ -404,6 +404,9 @@ class AssignmentCollection:
     def __eq__(self, other):
         return set(self.all_assignments) == set(other.all_assignments)
 
+    def __bool__(self):
+        return bool(self.all_assignments)
+
 
 class SymbolGen:
     """Default symbol generator producing number symbols ζ_0, ζ_1, ..."""