PPSD: add a switch for hydrophone data

... and unify with special handling of rotational data (from ringlaser).

I was asked whether we could add support for hydrophones (omitting the differentiation in processing), so here goes. There could be better / more canonical ways to do this.. any other suggestions welcome. One downside of unifying the handling of ring laser data could be problems when loading an old pickle. I'm trying to avoid those problems but this should be tested..