Support importing 2D Nicos data files

Loading shall be available via GUI and via Core, same as e.g. for TIFF right now.

File extension is "*.001".

Along with this, the open-file-dialog filters shall be divided in these parts:

TIFF (*.tif, *.tiff)  
Nicos (*.001)  
BornAgain (*.int.gz)  
ALL (*.*)