[BM29] Correct formula for standard error of mean

[kif]

For this:

• record sum_signal, sum_norm and sum(ω'²) into HDF5 at the integration level.
• export sigma = sqrt(sum(sum_signal)/sum(sum_norm)) possibly mulitplied by sqrt(sum(ω'²)) (to get the SEM instead of the STD)

[kif]

For sure, we are speaking of std, not sem