Pdet dependencies

updated the dependencies in eventgen.transforms.Pdet to only rely on net_snr, meaning eventgen.transforms.SNR must have been appended to each EventGenerator before Pdet. This is done so we can re-compute the pdet attribute with different SNR thresholds without having to re-generate the waveforms (or know the network) for each event.

There is a corresponding change in exposure here.

changed the description

@chris-pankow, I opened this mostly to make sure you saw it. I'm going to go ahead and merge it in since it's a pretty small change (and I've already updated other docs to reflect the new behavior.

merged

mentioned in commit 62fbc3f0