#3 NiftyRec.py missing global BG_ACTIVITY, BG_ATTENUATION, EPSILON

[Matteo]

Hello,

the Reconstructor class from NiftyRec.NiftyRec is missing the BG_ACTIVITY, BG_ATTENUATION and EPSILON variables.
It works if you use the Reconstruction.py version, since it defines these variables.

python simple_phantom_example.py 
The test phantom volume is of size(720, 1, 720)
Traceback (most recent call last):
File "simple_phantom_example.py", line 55, in <module>
    r = Reconstructor(volume_size)
File "/usr/local/lib/python2.7/dist-packages/NiftyRec/NiftyRec.py", line 262, in __init__
    self.background_activity = single(BG_ACTIVITY)
NameError: global name 'BG_ACTIVITY' is not defined