[IPOL discuss] IPOL demo (a bit) simpler

[Miguel Colom]

Dear all,
I've implemented in the base class base_app.py the following methods:
- select_subimage
- rectangle methods

This means that they are no longer needed in the app.py of new demos.  
The idea is to simplify the construction of new demos, and to avoid  
having duplicated code.

These methods seems to be duplicated in every demo subclass, and  
therefore they have been moved to the base class.

For the demo already published, nothing must be done. They will  
contain duplicated overriding code, but it's not really a problem.

For new demos you might be writing, simply get rid of those two  
methods. Your code should work as usual, but simpler. The new file can  
be obtained from git, or simply from  
http://dev.ipol.im/~colom/base_app.py

All the best,
Miguel