better imports

I think
Correction ?

Not sure it's what should have been done
remove pandas option in translate_frequency

As it's now a method of TimeArray (based on numpy)
sali_for_regime instead of plaf_sali
correction + _
oups
try to remove calculate_SAM

huge need for comments
some rewrriting of sum_sam

I have no idea what it does...
again one undescore