DESCRIPTORS: Completely split operator() and actual process method

This is part of #39 and allow easy usage through the manager while keeping
the ability to test each class.

Signed-off-by: G4m4 <misept.dieseneuf@gmail.com>
MANAGER: Move apodizer into the manager

Part of #37.

Signed-off-by: Gama <misept.dieseneuf@gmail.com>