Return detected indices

[jacopok]

Change the signature of compute_fisher_errors to

detected, snr, errors, sky_localization = compute_network_errors(...)

where detected is an array of indices for the detected signals, while snr and the other return values contain all the signals, even the ones that were not detected.

This solves a bug with the saving of the data in cases where not all signals are detected.

[u-dupletsa]

check OK