smurf: Add RTS delay interpolation

Due to a delay discovered between the real time signal (RTS) timing pulses
and the corresponding mirror positions of ~6.3 ms,
interpolation of raw interferograms has been added.
An optional parameter: RTSDELAY accepts a double value representing time in ms.
smurf: Rewrite RTS values for reverse scans to be increasing in time
- The GSL interpolator requires monotonically increasing values for the X dimension
  so the first reordered RTS timestamp is set to the time of the first original timestamp
  and subsequent timestamps are incremented by the original difference between successive values