Check coupling of GWs

We currently couple the GWs (expected in fractional frequency deviations) in carriers and sidebands using self.central_freq,

logger.debug("Propagating carrier fluctuations to distant MOSAs")
self.distant_carrier_fluctuations = ForEachMOSA(lambda mosa:
    self.central_freq * self.gws[mosa] + (1 - self.d_pprs[mosa]) * \
    self.interpolate(self.local_carrier_fluctuations[ForEachMOSA.distant(mosa)],
        -self.pprs[mosa] * self.physics_fs)
)

logger.debug("Propagating upper sideband fluctuations to distant MOSAs")
self.distant_usb_fluctuations = ForEachMOSA(lambda mosa:
    self.central_freq * self.gws[mosa] + (1 - self.d_pprs[mosa]) * \
    self.interpolate(self.local_usb_fluctuations[ForEachMOSA.distant(mosa)],
        - self.pprs[mosa] * self.physics_fs)
)

I'm not sure those are the correctly coupling factors (see #17 (closed)). To be checked with experts.