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

-from pystencils.simp import (SimplificationStrategy, insert_aliases, insert_constants, insert_symbol_times_minus_one,
+from pystencils.simp import (SimplificationStrategy, insert_constants, insert_symbol_times_minus_one,
                              insert_constant_multiples, insert_constant_additions, insert_squares, insert_zeros)
 
 
@@ -16,6 +16,5 @@ def _default_simplification():
     s.add(insert_squares)
     s.add(insert_zeros)
     s.add(insert_constants)
-    s.add(insert_aliases)
     s.add(lambda ac: ac.new_without_unused_subexpressions())
     return s