Following olivier's suggestion, I refractored the code that retransforms
the output in either a list or tuple

We are not importing scan directly anymore

the previous code returned list of tuples (not 100% sure why - I guess is a
python thing). This makes sure we are dealing with a list of variables.

Fix tests sandbox scan

There are two things to wait for :
 1) Fix bugs in sandbox scan
 2) Add more restriction to scan which would simplify this test

avoiding name collision and other things

also make sure it has the right sign

initial state can be one dimension less if the taps equal just [-1]