changes to /config/inspiral.py to handle empty snr_optimizer section of config, and to dags/layers/inspiral.py to only use snr_optimizer section of config

Added snr_optimizer.ifos

The sum of noise counts may change due to first-round-extinction, so this commit stores the sum of counts in the PDF before extinction in a different variable, and this variable is used for checking the health of the PDF (which is exactly what was being done before)

marginalize_likelihoods_online: Write noise PDF only if more than 99% of bins have undergone first round extinction

Correctly save the backup zerolag PDF in the backup dir within the analysis dir; Also save the backup diststat PDF there

1. It modifies the create_prior_diststats layer to use the newly created prior diststats of the first bin only as a bootstrap to create an empty zerolag pdf. This means that the empty zerolag pdf is created during the setup dag, not during the online analysis.
2. It modifies the trigger counter to fail if it cannot access this zerolag pdf, instead of doing the bootstrapping procedure again. This makes it so that if the zerolag file is inaccessible, the trigger counter will fail instead of assigning wrong fars and uploading wrong events.
3. It modifies the trigger counter to regularly save a backup of the zerolag pdf with the current gps time in the filename. This is done so that if the actual zerolag pdf gets lost/corrupt, we can easily restore the latest backup instead of trying somehow to reconstruct the zerolag pdf from scratch.