-------------------- Make plot of snapshots ------------------------------
 
  Call: data=image_snapshots(sample_rate,Nplot,Ntrn,Nrow,gain,dx,dt,field);
 
  * sample_rate: The rate of time-steps at which snapshots are written to
    the disk during the FDTD forward modelling
  * Nplot: The number of snapshots to be plotted starting from 1, or the interval of
    snapshots to be plotted given as a vector.
  * Ntrn: The transmitter number (starting with 0) from which the snapshots
    are plotted
  * Nrow: Numer of plots/row
  * gain: A gain factor used to amplify the snapshots as function of the
    snapshot number. The amplification is obtained by scaling the initial
    color axis with the factor g=(i*(gain+1))^-2. For a gain factor of 0
    now amplifcation is applied to the snapshot.
  * dx: The cell size (only needed to obtain the correct axis in the images)
  * dt: Used to determine the time of the snapshots
  * field: The direction of the eletrical field 1: Ez, 2: Ex 
 
  Knud Cordua (2008)
 
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