ACTIVE_MG probably wrong

About the following change

from bliss.common.measurementgroup import get_active as get_active_mg
ACTIVE_MG = get_active_mg()

Be careful @muzelle . It's probably not the ACTIVE_MG, but only the active MG at the time you call the function.

So if somebody change the active MG, this ACTIVE_MG will be wrong.

What you have to use is probably something like

ACTIVE_MG = session.active_mg()

So maybe something like:

current_session.active_mg()