Generalize the waveform writer to store floats

Currently the rwf writer stores data as int16. This is the correct option for the raw waveforms, but not for the corrected waveforms. We haven't been using Isidora enough to realize this, but the data it stores is (silently) cast from float64 to int16, which is not the correct behavior. I propose that the rwf writer stores int16s by default, but with the option of switching to floats*. This way, we don't need to change most of the code that uses this function.

*we will need to discuss which precision to use, but that's not critical.