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/optimizations.py

+"""
+Default Sympy optimizations applied in pystencils kernels using :func:`sympy.codegen.rewriting.optimize`.
+
+See :func:`sympy.codegen.rewriting.optimize`.
+"""
+
+
+try:
+    from sympy.codegen.rewriting import optims_c99
+except ImportError:
+    optims_c99 = []
+
+
+optims_pystencils_cpu = [] + list(optims_c99)
+optims_pystencils_gpu = [] + list(optims_c99)