Correctly print sp.Abs in generated code

Another issue reported by @alexander.reinauer.

pystencils/backends/cbackend.py

@@ -361,6 +361,12 @@ class CustomSympyPrinter(CCodePrinter):
         result = super(CustomSympyPrinter, self)._print_Piecewise(expr)
         return result.replace("\n", "")
 
+    def _print_Abs(self, expr):
+        if expr.is_integer:
+            return 'abs({0})'.format(self._print(expr.args[0]))
+        else:
+            return 'fabs({0})'.format(self._print(expr.args[0]))
+
     def _print_Type(self, node):
         return str(node)