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_tests/test_sum_prod.py

@@ -29,7 +29,7 @@ def test_sum():
         x.center(): sum
     })
 
-    ast = pystencils.create_kernel(assignments)
+    ast = pystencils.create_kernel(assignments, sympy_optimizations=[])
     code = str(pystencils.show_code(ast))
     kernel = ast.compile()
 
@@ -57,7 +57,7 @@ def test_sum_use_float():
         x.center(): sum
     })
 
-    ast = pystencils.create_kernel(assignments, data_type=create_type('float32'))
+    ast = pystencils.create_kernel(assignments, data_type=create_type('float32'), sympy_optimizations=[])
     code = str(pystencils.show_code(ast))
     kernel = ast.compile()
 
@@ -88,7 +88,7 @@ def test_product():
         x.center(): sum
     })
 
-    ast = pystencils.create_kernel(assignments)
+    ast = pystencils.create_kernel(assignments, sympy_optimizations=[])
     code = str(pystencils.show_code(ast))
     kernel = ast.compile()