Exceptions when dealing with bob::python::{const_,}ndarray and blitz++ array's are inconsistent

Created by: laurentes

When defining the python bindings, we have to perform conversions between bob::python::{const_,}ndarray and blitz++ array's. Sometimes a check (with a python runtime_error exception) is performed before calling the bz<>() method on the ndarray, sometimes not. We should define what the developer should do and update the python bindings accordingly.

Once decided, the coding convention wiki should be updated: https://github.com/idiap/bob/wiki/Bob-Coding-Conventions