#2 NMO should allow constant velocity

[seismick]

NMO can only take velocities from a velocity file or velocity trace. Every other seismic processing package that I used also allows the NMO module to
apply a constant velocity to whole traces, which is specified in parameters or taken from a trace header. This is quite useful.
I think it should be fairly simple to add such an option.
To get this at present, one would have to pick a single velocity location, then edit the resulting velocity file to be just a single velocity.