Refac save_objects

The idea is to simplify save_objects and data, but also to remove obj.param_name and prefer obj["param_name"].

In __getitem__, instead of using getattr we could allow only accept keys found in App.parameters or App.data.
I could remove the data property and use it like we use parameters.
In save objects we can update the data dict with app outputs like ReadImageInfo stuff, using GetParameterRole.

As you said we should stop to use App.param_name and prefer App['param_name'] instead. This should fix any problem with class attribute names. And #72 (closed) by the way
Also I'll move __getitem__ to OTBObject, because we want Output slicers to work !
We can allow get attr by name in App only.