Correction for Rest Frequency

In the case of multiple spectral line observations, users might want to change the rest frequency corresponding to the interested line transition.

1. Reset restfreq while loading the data:

smalod in=mirdata out=miriad.uv rxif=1 restfreq=345.795991 sideband=1

2. Reset restfreq with puthd:

puthd in=miriad.uv_rx1.usb/restfreq value=345.795991 type=double