force all double internal precision for higher order waveform decompression

[ahnitz]

Use double precision for (expecially) the higher order interpolation, even if the input / output is single precision. This ensures consistent accuracy. We are seeing for the higher order interpolation that the waveform can run into numerical issues with the phase calculation. It's still a largely memory bound calculation, so this doesn't seem to impact performance much and avoids the headache.

No issues seen with the existing linear order.