Address #13: Use sympy.codegen.rewriting.optimize

It's really comfortable to write optimizations in terms of sympy.codegen.rewrite.RewriteOptim:

    # Evaluates all constant terms
    evaluate_constant_terms = ReplaceOptim(
        lambda e: hasattr(e, 'is_constant') and e.is_constant,
        lambda p: p.evalf()
    )

This PR adds a parameter sympy_optimizations to the create_*_kernel functions that applies the list of optimizations to the assignments before creating the AST.

sympy.codegen.rewrite already has some optimizations. Some similar to the optimizations of pystencils. For example create_expand_pow_optimization(limit) is really similar to the logic in CustomSympyPrinter._print_Pow.

See #13 (closed)

Problem: old versions of sympy (e.g. from ubuntu CI) don't have sympy.codegen.rewrite. The optimizations are skipped in that case. test_and_coverage applies all optimizations.

We could also try to implement a fma-optimization (fused-multipy add) with that and sympy.Wild.

pystencils/astnodes.py

@@ -509,6 +509,13 @@ class SympyAssignment(Node):
         self.lhs = fast_subs(self.lhs, subs_dict)
         self.rhs = fast_subs(self.rhs, subs_dict)
 
+    def optimize(self, optimizations):
+        try:
+            from sympy.codegen.rewriting import optimize
+            self.rhs = optimize(self.rhs, optimizations)
+        except Exception:
+            pass
+
     @property
     def args(self):
         return [self._lhs_symbol, self.rhs]