no-Alex configuration, histogram fails to calculate due to the missing trace.fret

[AI-rrr]

if one uses no-alex configuration, histogram will not be calculated due to the missing trace.fret variable.
solution is to calculate it in any case:

(
trace.acc.int,
trace.acc.bg,
) = lib.imgdata.tiff_stack_intensity(
vid.acc.raw, *masks_red, raw=True
)
trace.fret = lib.math.calc_E(trace.get_intensities())
# Acceptor (if FRET)
if vid.alex:
#we need to have trace.fret calculated in any case
#trace.fret = lib.math.calc_E(trace.get_intensities())
trace.stoi = lib.math.calc_S(trace.get_intensities())
else:
# If not ALEX, discard previous vid.red data