python wrapper. Pre/postcall can now be inlined

- Add the possibility to give a function "on the fly"
  for the precall and postcall of a python wrapped functoin.
- No change for previous code, it is a simple new feature.
- changed test accordingly. See my_module_desc.py for an example.
Fix #98

- A silly copy paste error : the operator - for matrix expression was
  defined twice.
- second error : a typo in unary method.