Refactored NITF feed parser so we can modify data mapping in settings
Refactored NITF feed parser so we can modify data mapping in settings

Data mapping is now done in dictionnary, which can be overriden in
settings with NITF_MAPPING parameter.

Mapping dictionnary operating is explained in _generate_mapping docstring.

This has been done to implement specific behaviour requested by the NTB
project.

SDNTB-201

mdhaman/superdesk-core,nistormihai/superdesk-core,superdesk/superdesk-core,ioanpocol/superdesk-core,hlmnrmr/superdesk-core,nistormihai/superdesk-core,ioanpocol/superdesk-core,ioanpocol/superdesk-core,ancafarcas/superdesk-core,petrjasek/superdesk-core,ancafarcas/superdesk-core,marwoodandrew/superdesk-core,mdhaman/superdesk-core,mugurrus/superdesk-core,petrjasek/superdesk-core,superdesk/superdesk-core,superdesk/superdesk-core,mugurrus/superdesk-core,mdhaman/superdesk-core,petrjasek/superdesk-core,mugurrus/superdesk-core,hlmnrmr/superdesk-core,superdesk/superdesk-core,marwoodandrew/superdesk-core,petrjasek/superdesk-core