Distinguish between SymPy and pystencils Assignement better

This MR clears the usage of SympyAssignement in contrast to the sympy assignement.

In the backend of pystencils only SympyAssignements are used now that inherit from pystencils base Node class

fixes #61 (closed)

Additionally, AddAugumentedAssignement is introduced for convenience

pystencils/backends/cbackend.py

@@ -262,19 +262,20 @@ class CBackend:
         return f"{prefix}{loop_str}\n{self._print(node.body)}"
 
     def _print_SympyAssignment(self, node):
+        printed_lhs = self.sympy_printer.doprint(node.lhs)
+        printed_rhs = self.sympy_printer.doprint(node.rhs)
+        binop = node.binop
+
         if node.is_declaration:
             if node.use_auto:
-                data_type = 'auto '
+                data_type = 'auto'
             else:
+                data_type = self._print(node.lhs.dtype).replace(' const', '')
                 if node.is_const:
-                    prefix = 'const '
+                    data_type = f'const {data_type}'
                 else:
-                    prefix = ''
-                data_type = prefix + self._print(node.lhs.dtype).replace(' const', '') + " "
-
-            return "%s%s %s= %s;" % (data_type, node.binop,
-                                     self.sympy_printer.doprint(node.lhs),
-                                     self.sympy_printer.doprint(node.rhs))
+                    data_type = f'{data_type}'
+            return f"{data_type} {printed_lhs} {binop}= {printed_rhs};"
         else:
             lhs_type = get_type_of_expression(node.lhs)  # TOOD: this should have been typed
             printed_mask = ""
@@ -350,7 +351,7 @@ class CBackend:
                     code += f"\nif ({flushcond}) {{\n\t{code2}\n}} else {{\n\t{code1}\n}}"
                 return pre_code + code
             else:
-                return f"{self.sympy_printer.doprint(node.lhs)} {node.binop}= {self.sympy_printer.doprint(node.rhs)};"
+                return f"{printed_lhs} {binop}= {printed_rhs};"
 
     def _print_NontemporalFence(self, _):
         if 'streamFence' in self._vector_instruction_set: