Fix symbol counters

When simplifications are applied on an AssignementCollection that is created with Assignments coming from another AssignementCollection that was simplified before the counter for the symbol creation was not respected

pystencils/simp/assignment_collection.py

@@ -61,8 +61,11 @@ class AssignmentCollection:
 
         self.simplification_hints = simplification_hints
 
+        ctrs = [int(n.name[3:])for n in self.rhs_symbols if "xi_" in n.name]
+        max_ctr = max(ctrs) + 1 if len(ctrs) > 0 else 0
+
         if subexpression_symbol_generator is None:
-            self.subexpression_symbol_generator = SymbolGen()
+            self.subexpression_symbol_generator = SymbolGen(ctr=max_ctr)
         else:
             self.subexpression_symbol_generator = subexpression_symbol_generator
 
@@ -453,8 +456,8 @@ class AssignmentCollection:
 class SymbolGen:
     """Default symbol generator producing number symbols ζ_0, ζ_1, ..."""
 
-    def __init__(self, symbol="xi", dtype=None):
-        self._ctr = 0
+    def __init__(self, symbol="xi", dtype=None, ctr=0):
+        self._ctr = ctr
         self._symbol = symbol
         self._dtype = dtype