Resampling Propagator does not re-initialise `spectral_axis`

[rob-shalloo]

After the recent changes to the handling of the spectral axis within Grid, there seems to be a new bug. If you perform a resampling propagation the spectral_field_valid remains True, but the spectral_axis is not copied across to the new Grid.

The ultimate effect is that if you run a resampling propagation, you cannot then manipulate the pulse further.

The fix is quick, but given the imminent change to the new propagators, we might not want to implement a new release before that.