Get correct noise weights in mapmaking when performing noisy runs

[JulienPeloton]

Currently, TOD weights are set to one (i.e. no noise weighting while performing mapmaking). While this is OK for noiseless runs, one would like to weight properly the TOD if one uses noise. There are several ways to weight the timestream, but one could simply use the PSD of the timestream (routines already exist). The problem currently is that it adds a bit to the runtime (precomputation and store on disk for MC runs?).