Introduce default assignment simplifications

There are many simplifications in pystencils that are usually not applied. They can be applied now by using a simple argument in the configuration.

Fixes #39 (closed)

pystencils/simplificationfactory.py

@@ -3,6 +3,11 @@ from pystencils.simp import (SimplificationStrategy, insert_constants, insert_sy
 
 
 def create_simplification_strategy():
+    """
+    Creates a default simplification `ps.simp.SimplificationStrategy`. The idea behind the default simplification
+    strategy is to reduce the number of subexpressions by inserting single constants and to evaluate constant
+    terms beforehand.
+    """
     s = SimplificationStrategy()
     s.add(insert_symbol_times_minus_one)
     s.add(insert_constant_multiples)